why do I need to go to 252? Is it just the way that the numbers add up?
Yes. For every 4 evs it is one point in the stat. 250 does not divide by 4 evenly as 252 divides evenly into 63. That increases the stat by 63 points. You can do any amount of evs as long as it divides by four.
Hope you found everything you're looking for here! There's a guide to EV Training on PC here (although the Pokemon areas are different in B2W2, the mechanics are the same), and if you have any further questions feel free to ask in the quick questions thread in the section for the game you're playing. Or alternatively just shoot me a PM. :)