So, it's a lot harder to get Perfect Damage than it is to get the current Double Kill + Last Leg.
I'd suggest increasing the Last Leg by +5% since it's easy to get that, keeping Perfect Damage as-is, and then changing Double Kill to something like:
Double Tap (below -20% starting max HP): +5%
Overkill (below -50% starting max HP): +5% cumulative, for +10% total with above
Double Kill (below -100% starting max HP): +10% cumulative, for +20% total with both above
That way when you get the -100% Double Kill it will feel impressive (with a higher total bonus and more lines of text), and that seems appropriate since it's going to be more time-consuming to pursue.