Multiply HP by 100, then divide by the actual HP. Then divide the fractions of the bar by 100 (Not in RPG maker, of course), for example, a bar with 20 fractions has 20/100 = 5. Then divide the percentage you got by this and make show picture events for every number you got by this. If you divide by 5, for example, you'll have 20 different numbers. 20 = 100%, 19 = 95%, 18 = 90%, etc... That's it, pretty much. :s