If I'm correct then Tailwind increases the Speed of all Pokémon on your team by 2x for three turns, counting the turn you use it. This means that a Pokémon with a Speed value of 120 would have 240 Speed for the duration of Tailwind.
I want to give my Azumarill enough Speed EVs to out-prioritize BB from Talonflame with Aqua Jet under Tailwind.
- Azumarill's base Speed, with 31 IVs, 0 EVs and a neutral Nature is 70.
- Talonflame's base Speed, with 31 IVs, 252 EVs and a neutral Nature is 178.
Therefore, I need my Azumarill to reach a Speed stat that, when doubled by Tailwind, will reach at least 179 Speed. As this is not possible through doubling, he will need to reach 180.
180 ÷ 2 = 90
I need Azumarill to reach a Speed stat of 90.
Am I right?