[iDarkCara]

I'd rather have it lowered to 15% of missing hp than a flat rate.

Reason being @ 350hp with flat scale, you have +50 to hit if they had 700 hp.
Currently, that would put it at 87.5 hit.
@ 15% it'd be 52.5
Given then, that you have them from 500 total hp, @ 250, you now only gain 37.5 hit as opposed to flat 50.

Reason being, this gives a disadvantage to squishy dodgers, but still provides a good choice against those with a bit more hp to spare.