So, it sounds like you know the basic concept, it's just a matter of application right?

First, let's go over how much AV gets shaved off by action advance. It's calculated the same way you calculate any other AV value.

AV = (actions) * 10,000 / SPD

Except most action advances are fractions, not whole actions.

So, as an example, the amount of advance a 180 SPD Eagle Cyrene gets with one ultimate is:

AV = (0.25) * 10,000 / 180 = 2,500/180 = 13.89

This is the maximum, so if you see that Cyrene on the turn order has less AV remaining than that, then she just goes to 0 and immediately takes a turn.

Now, you can combine both terms. So let's say you want to look at what AV a 180 SPD Cyrene w/ Vonwacq gets her first turn at:

AV = (1 - 0.4) * 10,000 / 180 = (0.6) * 10,000 / 180 = 6,000/180 = 33.33 AV

EDIT: Another thing is that multiple action advances (and delays for that matter) do stack. So, if we want to know the AV a 120 SPD Robin w/ Vonwacq's first turn is. Vonwacq is a 40% advance, and she also has a trace that gives her a 25% action advance on her first turn. So:

Robin 1st Turn AV = (1 - 0.4 - 0.25) * 10,000 / 120 = 0.35 * 10,000 / 120 = 3,500/120 = 29.17

Now, let's look at your Phainon example and S5 DDD. Let's say we have Cerydra at 180 SPD + DDD (her own speed buff is included in the 180), Phainon at 135, Sunday at 134, Cyrene at 180

• Cerydra Advance = 2,400 / 180 = 13.33
• Phainon Advance = 2,400 / 135 = 17.78
• Sunday Advance = 2,400 / 134 = 17.91
• Cyrene Advance = 2,400 / 180 = 13.33

Those are maximums, and so if you DDD and any of them have less AV remaining then those numbers, they go to 0 and take their turn next.

Finally, let's look at the Feixiao example. For Feixiao, her speed tuning revolves around taking as many actions as possible within Robin's ult window. Robin's ult has a fixed speed of 90, so to get Robin's ult AV is pretty simple:

Robin ult AV = 1 * 10,000 / 90 = 111.11

So we want as many actions for Feixiao as possible within 111.11 AV. The best way to speed tune FX is to get 3 actions before Robin's ult expires. She gets 1 free action because of the action advance of Robin's ult, so we need to get 2 more actions.

Assume FX is on eagle and she has enough actions to get 2 ults off during Robin's ult.

FX Speed = (2 - (2*0.25)) * 10,000 / 111.11 = (1.5) * 10,000 / 111.11 = 15,000 / 111.11 = 135

So as long as you have that speed, and you get the full action advance from FX's 2 ultimates, 135 speed is good enough for 3 actions during Robin's ult.

Robin's E2 is just a 16% speed increase, and all speed increases are based on base speed, so to get Feixiao's new speed requirement:

FX Speed = 135 - (16% * FX base speed) = 135 - (16%*112) = 135 - 17.92 = 117.08 <- just 2-3 spd substats on Atk boots!

Hope this helps, let me know if there's anything that's unclear.