You don't assign a value... the ammount given it tied to your rep power and their rep power... someone with a high reputation giving rep to a low one will boost them by a lot, where as someone with less rep giving rep to someone who has a lot, it won't go up as much.
Which may be part of Pheadus's problem... you're rep is too damn high, it's probally going up by insignificant ammounts
Two posters, both with roughly equivalent rep as me, just gave me +rep, and it went up 1 point for one and zero for another.
So I doubt that is the reason....
Again, it doesn't matter to me, except I really love solving problems, and this will stick in my head until I know the answer...