To quote the damage itself, you need to have Superman as your dummy AI and enable the competition mode on in practice. Why? Here's the explanation.
The competition mode disables anything the gear wears and reverts the wearers to the basic stats. Superman's HP is exactly 1000 Health Points (HP), therefore, it's exactly 100%.
Now, how to quote it?
Every 10 HP consists only of 1%. If your combo dealt around 392HP, then it's exactly 39%.
The rest of how to convert the damage percent into another based of any character, I'll leave that to
@Lokheit .