size of a raise in a 10-handed CG?

I'm playing at NL100 where I usually raise 4xBB UTG with a decent hand such as AQ. Am I in a later position with a few limpers I cleary have to raise it up a bit more to get rid of the limpers (make the pot-odds less attractive).

What I am wondering is if there is any mathematical aspect behind all this and what it is, it would be nice to know so I could adjust my pre-flop raises to some formula or just know I am doing the right raise pre-flop.