Funny and amusing situations...

This is nonsense to me; it's enough to know that 1 attack increases damage by 5%, and 1 defense reduces it by 2.5%. Why make it so complicated?

Actually, 15% will only apply if attack and defense are equal. In all other cases, the percentage from Stone Skin will decrease, and in some cases, it can drop to 0%, meaning it can be completely useless. This is to keep it simple. And to make it complicated, you need to calculate it using the formula from Physics, which I think is too complicated for you.