SS AP buffs/Debuffs

Basically how damage is calculated in SS is that all AP buffs and HIGHEST AP debuff are tallied together and then applied to your base dribble (there's also positional bonuses but let's ignore that here)

For number crunching purposes
Assuming you have a base dribble of 800
you have the following AP buffs of - 25%/50%/30%
and an AP debuff of - 40%
this means your final dribble is calculated as 800 x (1 + .25 + .50 + .30 - .40 )
this amounts to a final Dribble of - 1320
so a 65% AP boost nets you a total of 520 Dribble
The 40% AP debuff is effectively negating 320  Dribble

Now let's look at Dribble Drains

We have Khirel, Miri and Damien who are the most prominent users of it (Felix's instances of dealing with AP drains are negligible and Bora's drain is as relevant as her presence in the metagame at the moment)

We'll only be looking at the Dribble part of the drains for these calculations, the operations however are the same in regards to steal but just uses steal stat instead of dribble

Firstly, Khirel
[Passive] Song of Knife II
Increases the damage that the target receives by 50% and steals 30% of the attack power when being attacked or with a penetration.

Let's say you are shooting khirel with the following stats
1200 base dribble
90% / 80% / 55% AP buffs for a total of 225% AP (Taking a massive AP buff to make up for a pass you'd normally receive before shooting to make things simpler to break down )

Now looking at how AP debuffs work previously,You'd assume khirel's 30% AP drain behaves like a 30% AP debuff that transfers the "debuffed" 30% to Khirel.
So your final dribble would be - 1200 x ( 1 + 0.9 + 0.8 + 0.55 - 0.3 ) = 3540 Dribble
Khirel drains 360 Dribble for himself

However, this is NOT how AP drains work
What actually happens is Khirel is a FLAT 30% drain applied to your FINAL AP
So therefore AP calculations will normally happen - 1200 x (1 + 0.9 + 0.8 + 0.55 ) = 3900 Dribble
and then Khirel Drains 30% - 3900 x (1 - .3) = 2730 Dribble
This means Khirel Drained 1170 Dribble

In AP debuff terms, for 1200 dribble you'd need to have a Debuff value of 97.5% to reduce as much AP as Khirel singlehandedly does

And then you realize AP debuffs Don't stack.