I have done this sort of thing before, it is best to throw out a trial balloon, let excel have at a couple of doz diff teams and adjust as ratings / results come in.
BUT, i think most ratings work by division, with the formula something like this for d1:
ATH for PG = (ATH-50)*.05 or speed (SP-70)*.15, etc
I have tried to use math to model ratings to performance, the trouble is without the opponent ratings for each game, your own team models only paint a part of the picture. For those who play strat o matic, i often theorize that offensive player ratings account for exactly one half of the touches a player makes, the defenders for the other half - not sure if that makes sense to others, essentially roll one red dice, if 1-3 comes up I look at the off guy, if 4-6 comes up I look at the defense. This explains why sometimes some pretty great guys / teams don't do so well, if they play off the defenders card an abnormal number of times??????