custom wheel offset

I am going with a 20x8.5 or a 20x9 wheel. Does anyone know about what my offset needs to be? Any help would be highly appreciated. Thanks!

Atleast +45 if you don't want to shorten axles or front ends. Just note that a 18x8 with a +45 that I have tucks easily but im running real close to the wheels rubbing on the frame in the back.