CUSTOM BATTLE SYSTEM INFO

Your question seems way too simple. Just use two Variables, set them to equal your hero's Attack and Defense stats, then do whatever operation you want to do and subtract from the target's HP.

I suppose this is for 2k3?
What kind of CBS do you want to make? A turn-based front view CBS? An ATB side-view CBS? A tile-based ABS?

Don't actually use the FF damage formula please. It is seriously extremely awful, the way defense works is just absolutely non-functional, the only reason you never noticed is because the games are all super easy except for one or two battles and so you never really paid attention to whether anything actually happened when your stats went up.

If you want someone to rant about choosing and balancing your damage formulas for a hundred pages I CAN TOTALLY DO THAT. Someone should move this to GD&T though if that's really what you want. Since it was posted in Help & Requests, I assume you want technical scripting/eventing help though.

OK, stat mechanics rant incoming. Someone move this topic to GD&T.

About half of this is copypasta from something I mailed to Link a month ago, but relevant to anyone trying to figure out how their stats will work. Maybe something I say will be helpful.

The standard Att/MAtt/Def/MDef/Speed is a pretty servicable set of stats. I have those five plus a Healing stat in my own game, because most of my characters only use either attack or magic power, only one or two characters use both, so there was very little choice regarding what stats to improve. By separating healing and magic (and giving the physical characters weak heals) I was able to create more of a choice when outfitting yourself. If you're using Lufia 1 and 2 as an influence, though, I'm guessing most/all your characters can proficiently use both magic and physical attacks, so you already have some choice there, if you actually include equipment choices that raise one or the other. Players like choices. It makes them feel like they're playing a game, instead of like everything they do is inevitable and they might as well just be watching it.

For the damage formulas, the big choice you have is whether you want defense to work subtractively or divisively. If defense subtracts damage from attacks, it's possible for attacks to deal 0 damage. This gives the players the ability to become invulnerable against weak enough enemies, and also means that low-attack-power characters will probably deal no damage whatsoever to harder enemies. It's possible to make attack power outscale defense by enough that this doesn't happen, but then defense points feel meaningless. If this kind of damage cut-off exists, players will base their strategies around it. If that's what you want to design for, use subtractive defense. You'll get that feeling of, YES, now with my newfound power, I can finally overcome this enemy's defense that I couldn't get through at all before.

On the other hand, if having twice as much defense makes the enemy deal half as much damage (or 25% less damage, or whatever ratio you use), the player will have a lot more ways to overcome challenges. If they're half as strong, they can still win by lasting twice as long. Their defense might be low if they haven't bought the latest armor or are a few levels behind, but it's probably never going to a quarter as high as it's supposed to be, so the enemy's never going to deal CRAZY damage based purely on your armor's poor def. Enemies can obviously still overpower you, but it's a softer cutoff. Overall this makes enemies, status effects and equipment vaaaaastly easier for you as the designer to balance. You can basically be pretty sure how much damage the player is going to be dealing and taking each round, within a range, which makes your job a lot easier. And if a buff increases power by 40%, then it always increases damage by 40% - whereas with a subtractive formula, it might increase damage by 50% against low-def enemies and 600% against high-def enemies. This makes stat buffs and debuffs feel like they're more equally useful all the time. You might or might not want this! I like this because it feels like even against weaker enemies, I get to use some fun skills and all the battles don't feel so samey. Other people might prefer the easier battles to just end ASAP, though.

Speed can be a weird stat in a turn-based system. If you have to input your entire party's actions before the turn starts, then your characters' speed compared to each-other can be a lot more important than their speed compared to the enemy. You want the guy with the offensive buff or the defensive debuff to go before the guy with the area attack. You might want the reviver to go last, after the boss has attacked, and the healer to go first on the next round, before the boss attacks again. If someone has an attack that hits 4 random targets, you always want them to attack last, so the other characters' area attacks hit more targets. So there are a lot of situations where the player will want to lower their speed. The best way to handle all of this is probably to let the player customize their stats - whether by some kind of stat-allocation each time you level up, or just by having a few accessories that alter speed by different amounts.

You can of course include tons of additional stats if you want. Hit rate, dodge rate, magic dodge rate, crit rate, crit damage bonus, status effect hit bonus, status effect dodge rate, HP regeneration, MP regeneration, elemental resistance penetration, spell reflect rate -- basically almost anything that can be a passive ability or a side-effect of a spell can be turned into a stat instead. These are good for... certain types of players. They're good for people who love the tactical aspect of RPGs and enjoy all the choices and micromanagement because it gives them tons of choices of ways to fight, and they're good for hardcore players who get a kick out of minmaxing their characters into gods. They make your game much more difficult to learn, though. You create gameplay situations like "OK, bonus crit chance is my best stat to improve right now for battles where I'm not using damage over time effects, but only up to 22%, after that attack power becomes better again. But the only way to add more crit chance right now is to remove this helmet that boosts my hit chance, so it'll drop below 100%, but I can resolve that by casting a hit bonus buff on the first round or by equipping this skill that has an increased chance to hit." But a lot of players, even most people who like RPG gameplay, will not really want to sit down and figure that stuff out. Even if they don't really have to learn it, they don't know they don't really have to learn it, and just looking at it overwhelms them. So you'll be driving them off. And if your game is easy, then all of that complexity and depth is wasted anyway because there's no reason for the player to bother with it. Why bother minmaxing your stats to deal 40% more damage in this dungeon, if you could just use normal attacks and healing spells to kill everything and still be in no danger?

Hit chance is worth talking more about specifically. Whether you make it into a stat the player can control or not, it has an extreme effect on your game. Any hit chance that's not 100% means your strategy has a chance to fail, through no fault of your own. If your strategy is "attack, attack, heal, attack" then who cares, right? Missing an attack will be a little frustrating, but battles are so boring that it's almost welcome at that point. And if your allies are all on auto-battle or you otherwise already can't predict major aspects of the fight, then a low hit rate just helps make battles a little harder occassionally and maybe adds an extra stat worth worrying about when you're equipping yourself. But if your strategy is "Use an area defense debuff with my fastest character, then use a strong single-target attack on the strongest enemy with my second character, then use an area attack with character 3, which with the previous two hits should be just barely enough to kill the strongest enemy before it has a chance to attack, then the remaining enemies should attack, and then I'll have my last character use his powerful area attack to kill all the remaining enemies" then suddenly missing one of those hits seriously screws you up, and might very well result in a game over. As a rule, if the player dies, it should be because he did something wrong. Not because you just decided to randomly kill him. So the more strategic your game's combat is, the worse idea it is to have any hit chance that isn't 100%.

Note that none of this hit chance argument necessarily applies to hit rates of enemies, though. If an enemy randomly misses, the player is relieved, not frustrated. They might have wasted a little MP, but nothing they did worked any worse than normal, and it might have just saved their butt. However it DOES apply to enemy crit rates. Not to the same extent, though. I find enemy critical hits are generally more or less okay for normal battles, where they can randomly make normal battles be almost as difficult as bosses, and terrible for bosses, where they can randomly make the bosses one-shot you. Meanwhile, player miss rates are actually worse for normal battles than for bosses, because in a normal battle you are dealing damage to avoid taking damage, by killing enemies before they can hit you. Whereas in a boss fight that's not usually the case unless there are multiple targets or some kind of timer, and each hit is only dealing 3-5% of the enemy's health, so the misses sort of average out over time. If you want to only give dodge and crit chances to non-boss enemies, that's certainly an option, and can help make normal battles less repetitive.