Astrologian buffs confirmed

[Ghishlain]

1.5% for each of 8 people, which is a cumulative increase of 12%. If we want to call it 1.5%, we can, we just have to then divide AST's 1.5% by 8 to get the actual number since we're only casting that buff on one person instead of 8.

No, this is incorrect.

Player 1 contributes 300 DPS, a 1.5% increase to their DPS is 4.50 DPS for a total of 304.50 DPS
Player 2 contributes 350 DPS, a 1.5% increase to their DPS is 5.25 DPS for a total of 355.25 DPS
Player 3 contributes 400 DPS, a 1.5% increase to their DPS is 6.00 DPS for a total of 406.00 DPS

Total DPS of the group pre buff = 300 + 350 + 400 = 1,050
Total DPS of group post buff = 304.50 + 355.25 + 406.00 = 1,065.75

1,065.75 / 1,050 = 1.015 or a 1.5% increase to group DPS.

You can't sum the parts of each DPS and assume their sum is the total increase to raid DPS without also including the sum of the base raid DPS before the buff. That's the error in your math.

Likewise, while Arrow is approximately an 11% boost to a specific player, that's 11% specific to that particular player. So use my previous example.

Player 1 contributes 300 DPS, a 11.1% increase to their DPS is 33.30 DPS for a total of 333.3 DPS
Player 2 contributes 350 DPS
Player 3 contributes 400 DPS

Total DPS of the group pre buff = 300 + 350 + 400 = 1,050
Total DPS of group post buff = 333.3 + 350 + 400 = 1,083.3

1,083.3 / 1,050 = 1.032 or a 3.2% increase to group DPS.

Remember in math, you always want to try to calculate like for like as much as possible.