For some reason the example was edited out, but Nana was referring to multi-attacks with boost.

I'm not sure of the exact formula behind it, though it has been tweaked over time so that boosts don't benefit AoE attacks 1:1, otherwise boosts would be 4x as effective against 4 targets, which is definitely overpowered.

Here's a combat log that may give people a better sense of just how much boost benefits AoE. (Edited just to show Destructobeam's boost)

Torq uses [Cleric Power Infusion]

Destructobeam's next magic attack is increased by up to 34

Destructobeam uses [Banishing Flame Stone]

Sasquatch takes 6 magic damage (27 warded)

Alastor takes 33 magic damage

Croissant takes 33 magic damage

Atomsk takes 33 magic damage

Banishing Flame Stone does 22 damage each, so that's 11 damage extra each attack against 4 players, a total benefit of 44 because of a 34 damage boost. (Bringing 88 total damage up to 132) A little bit better than 1:1, but this is only for 4v4.

Your formula fits this.

The power boost reduction on AE depends on the number of target.

Against 4 players, CPI + BFS will do 33 damage to all

Against 3 players the same charms will do something like 38

If only one enemy is alive, those two charms will make 54 damage, the buff being applied to 100% as if it was not an aoe charm.

But it does not fit this. 38 - 22 = 16. But 34 * 1.3 / 3 = 14.7, not enough boost. Schutz's formula does fit, though: 34/2 = 17 (not exact, but note Lez said "something like").

Schutz has a fight linked in the best fights thread as well where he has 905 boost, with 301 applying to each of 4 targets. 905 * 1.3 / 4 = 294.125, again not enough, but 904/3 = 301.33, exactly right.

The reason you don't often see fights where the * 1.3 version doesn't work is that in a 4 player fight, multiplying by 1.3 and dividing by 4 is almost exactly the same as dividing by 3, so you don't notice the difference unless there's a huge boost. The right formula is definitely the one schutz gave.