Vitamins(proteins,calcium etc.)can give you a max of 100 EVs, so it makes training shorter.
The power items give you 4EVs of that paricular stat(power wheight gives u 4 in HP, etc.) so its 4+a(the number of EVs the defeated poke gives)
Macho brace doubles the amount of EVs gained. if you defeat a crobat, who gives 3 spd evs, u get 6 with the macho brace. so its 2a
Pokerus gives the same effect as macho brace, 2a. You can also use it with other items, for example, you want to train speed. If your poke with pokerus defeats a crobat, which as you know gives 3 Spd EVs, and its holding a power anklet, you get a total of 14EVs in 1 battle. You get 3 from crobat, 4 from the power anklet, and u have 7. With pokerus, it becomes 14. so an expression could be (presuming all the Evs are for 1 stat) (a+4)2 a=amount of EVs from the defeated poke, added to 4 of the EVs from your power anklet, multiplied by 2 from the pokerus. Hope this helped, and good luck
Make your poke