Yes at level 50 the number of points gained will be half as much as they are at level 100. So if you have full EVs of 252, at level 100 that's 63 extra points in the stat at level 100, or 32 extra points at level 50. You still have the same number of EVs, they just give you less points at lower levels.
In LC, however, it is a bit different. Obviously on a simulator you have all the EVs already in your disposal and you don't have to worry about it. In the games though, you have to watch out that you don't over-level while training.
While EV training a Pokemon for LC you want to:
- Gain as many EV points per battle as possible
- Gain as few EXP points per battle as possible
To do this, you first of all might want to have a backup in case you mess up. Such as a clone or something similar.
You might want to use something known as the "Box trick".
>Each Pokémon has a Base EXP rate which determines how quickly it levels up. Depending on which category your Pokémon falls into, you will have a different amount of EXP to use before it reaches level 6. If you deposit a Pokémon in the Box, its stats will be recalculated and EVs taken into account (normally you would have to wait for a level up for stat recalculation). This is called the "Box Trick" and it means you can finish training while at level 5.
How to minimize the EXP gain:
- You will want to battle Pokemon that give as few EXP as possible when looking for EV yields. For example, Magikarp gives a really low amount of EXP and yields you 1 speed EV.
- Give the Pokemon you are trying to EV train an EXP share, that was the EXP it gets is halved.
This is the method I've found which sounds pretty effective if you ask me. :]
Hope it answers your question.