What if Twitch's Contaminate did %Missing Health Damage at 6 stacks of his passive?

So a lot of people like to compare Twitch's E to Kalista's E and for the most part are annoyed by how much more arbitrarily powerful Kalista's edition of the exact same ability tends to go for a lot more. It also doesn't help that she can stack hers indefinitely while Twitch has a limit, but what if we make that apparent weakness into a strength for Twitch?

Let's note that many Kalista players don't use their E as a full execute, as many like to utilize the slow effect to help them kite anyone chasing them down. It makes sense then that, stack per stack, Kalista's E does more damage than Twitch's E if we assume it's balanced around Kalista's ability to kite (and let's be frank here, it better be balanced around her kiting or else nothing makes sense). We also can't exactly buff Twitch's E stack per stack because it could overload his front loaded damage, which as the Assassin flavored ADC of the game, makes him especially dangerous if he could pop out of nowhere and gun down a squishy without even requiring the full 6 (4 with W) shots to deal that much damage. Not to mention, it causes a lot more collateral damage during team fights that may be unintended.

So, how about rewarding Twitch for getting to those full 6 stacks. It doesn't step into Rend's territory of being safe to activate preemptively, it helps keep his front loaded damage tame and capable of fighting back, and it shouldn't throw his team fighting over the edge unless the Twitch in question has built super specifically for it or if he has the best Twitch mechanics possible (getting 6 stacks on multiple members of the opposing team without dying as Twitch has ought to be rewarded, right?)

I suggest %missing damage because it requires either good teamwork (everyone contributing damage) and/or for Twitch to actually be a relevant damage dealer (as a definite Hyper Carry, I think it's fair that if he wants to do damage, he can't fall too far behind). From what I can guess at, a flat damage bonus for hitting 6 stacks would be most effective against squishies, highlighting Twitch's Assassin trait but not doing much to play on his current mark of a hyper carry. A %AD bonus of any kind would probably just throw his snowballing out of control while not helping at all while behind, so I think that sort of idea should be avoided.