I'm currently working on a server in Minecraft which uses RPG/MMO gameplay aspects, I know you can change the arrow damage of a bow by adding the Power enchant, but I need to know how the higher levels of the Power enchant effect the bow, basically I need to know how much damage the bow does with for example Power III or Power IV, I checked the Minecraft Wiki but I found it hard to understand.
I checked the wiki and I hope my explanation is of better understanding to you. So basically at full charge on a regular un-enchanted bow you're looking at a about 10 damage (5 hearts).
The Power enchantment increases your arrows damage by a certain amount depending on what level the enchantment is, the maximum is level 5. As you progress through each level, the formula for how much damage the damage is increased remains the same: 25% × (level + 1). That increment is always based off of the original damage of the bow, never the previous level.
For example, an un-enchanted bow does 10 damage at full charge. I then enchant this bow to Power I, so I calculate the formula: 25% x (1 + 1) = 50% ; 10 + 50% = 15. My bow now does 15 damage (7.5 hearts) at full charge. Note: all damage is rounded to the nearest half-heart, e.g. with Power II you get 17.5 damage by using the formula, but since that can't happen in Minecraft normally, they round it to 18 damage (9 hearts)
There is a key available on the wiki you mentioned, perhaps you forgot to hit the expand button? I'll link it here.