Well, depending on the moves in the set you're running the EVs would vary. If you're for example, running 3 Special Moves (EG. Hydro Pump, HP Grass, Ice Beam) and 1 physical (Megahorn), just slap all the EVs in something like 4 Atk/252 SpA/ 252 Spe with Mild/Rash Nature
Since you're probably running a set like Hydro Pump/ Ice Beam/ Megahorn/ Aqua Jet right now... Obviously, your most powerful moves are in the form of special attacks, so maybe something like 64 Atk/ 252 SpA/ 192 Spe with Mild/Rash Nature.