Kai'Sa Caustic Wounds (Auto Attack Passive) Question

>Caustic Wounds - Kai'Sa's basic attacks stack Plasma for 4 seconds and deal [(7-12) + 10 % AP] + ([(1-9) + 5% AP] per stack) bonus magic damage. Kai'Sa's attacks detonate Plasma at 4 stacks, dealing bonus damage equal to ([15-20] + [2.5 per 100 AP])% of the target's missing health (max: 400 against monsters).

I see that it says detonate but I'm not 100% clear on it the order of operations on damage calculations. Does the 4 stack damage apply before detonating or if the detonating damage takes place in lieu of the plasma damage.

AA + Passive on hit + Stacking on hit + % missing or is it AA + Passive on hit + % missing

I'm trying to think of people who have on hit stacking damage and popping damage but I can't think of any. Tahm deals damage on every hit and there's no popping extra damage based on the stacks Braum, Kindred, and Vayne only damage upon popping the stacks with an extra auto attack but scale Varus' stacks scale but pop on abilities

Anyone on the PBE able to offer any insight?