My cousin and I are currently in the process of designing our own RPG. We have some basics for story down, some battle mechanics, and even some character's down, but we need a Damage Formula. We already designed stats for the equation (and the parameters), but every time we mess with the equations we got some errors.

• The damage becomes negative if the damage is "too high." So, when fighting a level 10 monster with low defense when you're level 5 would result in -20 damage or something along those lines.
• The damage becomes large too quickly. For example, a level 10 character might do 80 damage but a level 15 might do 300. It gets well above our intended Damage cap before it's supposed to.
• The damage gets too low at higher levels. This occurs when we try to map out our boss battle. Even when the stats are maxed out, you're only doing 100 and some damage to a creature that's supposed to have a large amount of HP, resulting in a drawn out battle (lasting more rounds than it should).

We've tried messing with equations from various games including Final Fantasy, Persona, Dragon Quest, and some of our own as well. In all cases it seems to always find an error, and when we try to fix it the damage dealt is about the same for all attacks.

By the way, we know you shouldn't be doing max damage when you face the boss at level 60 or something (intended endgame level for casual players). We intend the Max damage to be 999, and we intend the player to do around that, if not more, once they are around LVL 85 to 90 with the right equipment.

The stats we want to use in this equation consist of

• Level (LVL) - Kinda self explanatory. Max is 99, can be increased to 150 with certain items.
  
• Attack (ATK) - The combined value of Strength (STR) plus equipment bonuses. Max is 255, can be increased with certain items.
  
• Defense (DEF) - The combined value of Vitality (VIT) plus equipment bonuses. Max is 255, can be increased with certain items.
  
• Power (POW) - The power of the attack. The normal value is intended to be 16, but this variable is more so physical attacks that are meant to be stronger will be, so it can be changed.
  

Does anyone have some ideas to help us out? At least something to give us inspiration?

Hi and welcome! (don't be alarmed, I'm harmless)
 
Looks like you're working on a computer RPG? This site is largely for pen-and-paper RPGs, but you might still be able to find someone who can help (Peregrin or J Arcane, perhaps).
 
I don't know if its possible for you to post the equation you're using?
 
 
The too high damage becoming negative is weird and sounds like a programming error of some kind. The second and third problems sound like you just need to adjust the multipliers in the equation to work - and perhaps alter the target's hit points in parallel with that ?  
 
For the last problem, at worst, you could perhaps run some sort of counter for 'hits' separate to damage so that a target dies after a few rounds, though that makes a character's damage bonuses irrelevant and so may not be a good idea, and might be tricky to program.??
 
Regards,
BSJ.

BSJ is right.

You might want to try retrofititng the formuale. So start with how much damage you want to achieve at each level and then create the formula that gives you that output.  

Obvious but as BSJ says without the formula not a lot anyone can do.

Figure out a base damage cap per level. This should be a linear scale.

1 dmg / lv.

Thus, a level 10 character with a 1 attack power fighting a creature with a 1 defense power would deal 10 damage.

Now adjust for attack and defence:

Level * (Attack / Defence)

Thus, a level 10 character with 10 attack power fighting a creature with 1 defence will deal: 10 * (10 / 1) = 100dmg.

You can add additional factors to this, but that gives you a starting point I think.

Adjust character and creature HP totals to adjust the difficulty level of the game.

A character fighting in a Lv 5 dungeon is going to encounter creatures with approx 5 defense and 50 HP? Give them more HP to make it harder, less to make it easier. Also adjust enemy attack power to make them tougher or weaker, and enemy defense to make them tougher or weaker.

Damage types could be added in on a percentage basis.

Fire weapon deals 50% fire and 50% physical. Enemy strong against fire would take 1/2 damage, etc.

Hope this helps get you started.