How do I use percentages in expression stuff?

The title may not make much sense. Basically, I'm making a game where you have a chance to critical attack, but I'm not just making it deal X2 damage. It deals a percentage of damage that you can upgrade. When you start the game a critical strike will deal 200% damage, so it is then X2. But I want it so you can upgrade the percentage it deals. With a normal attack I just do 'Subtract player attack damage from enemy health'. How do I make it 'Subtract 200% Player attack damage from enemy health'. Or any percentage.

You could divide the damage amount by 100 then multiply by the percentage, so if damage was 60 and it was a 200% critical strike = 60/100=0.6*200 = 120. Later if a crit strike does 300% damage then 60/100=0.6*300 = 180.

That should work. It'll also work with the upgrading by using my global variable CriticalDamage in the event. Thanks!