THENECROMANCER'S PROFILE

The leveling system in FFxi is one that sounds like what your talking about.

To get the most EXP from a kill you have to defeat something of a higher level than you. As your level begins to match that of the monster you get less EXP. Eventually you will get no EXP from a monster. There is no "pool" of EXP, once you gain a level your EXP is 0(+ whatever is left over from the kill) and then your goal for the next level is raised by a few hundred.

Is this what your trying to do? If you could be specific about something your trying to accomplish I could toss some ideas around. Like how to create the branches to compare levels and alter the EXP gained.