This excel workbook helps you to determine what values to use for the "league totals" in the "advanced setup" screen in OOTP.
I've made a few versions of this in the past, but I think this is the simplest to use and most accurate version yet.
You can download the 409KB (approx.) zip file at:
http://members.shaw.ca/kq76/ootp/
P.S. If you're a perfectionist with your ootp leagues, then you'll probably at least want to try this out. If you're not, then don't bother as it can take some time to sim multiple seasons.
EDIT: If you're looking for a settings calculator for historical leagues, then you'll want IatricSB's Era Calculator. Mine is more for fictional leagues. I don't know if he has 1 out for ootp5 yet, but I remember him saying he was working on it.