What kind of growth you want depends entirely on your system and the effect you want levelling to have. So, uh, I can't help you there since I know nothing about the game.
About the 'plugging numbers in' thing, though -- if you don't want to come up with an exact formula (or if you want to use a bunch of different formulae and don't want to explicitly define all of them) I recommend zaril's
trimods.