simple question, simple answer.
that being said
i figured this out on my own when i first caught pokerus through simple math and logic, (albeit i guess i could have asked)
Magikarp = (1 speed EV + 4 speed from power anklet) x2 from pokerus = 10 EVs
so, effectively, if you match the power item with the EV that the pokemon gives, it makes 1 go to 5 go to 10.
i tested by battling 10 magikarp and then testing if it could eat another carbos. (it could not, meaning that it had gotten 10 or more EVs from each battle, logically, the max cap of 1 + 4 x 2 at any point is 10, given parentheses.)
if you wanted to battle magikarp and use a Power Item for, say, Sp. Def, it would go like:
Magikarp = (1 Speed EV + 4 Sp.Def EVs) x 2 Pokerus. = 2 Speed, 8 Sp. Def