What do you want to know?
If you want absolute force values I wouldn?t trust anything but full vehicle transient modeling, and even then I wouldn?t trust it without windtunnel and coast down correlation.
If you just want Delta?s of part changes then the nature of the part and it?s place will have an influence on the choice of cfd modeling.
I?m afraid there is no easy answer. People dedicate their entire careers to trying to figure this stuff out (myself included). Unfortunately many racers like to believe the first or cheapest answer they get, which is often misleading or just plain wrong.
I?ll be at Bonneville over the first weekend this year (unfortunately we can?t race this year), seek me out to talk (too much info for a thread). PM me for my cell number, that?s an open invitation to anyone.
Advice is free, implementation will cost you 😉