I have been through this several times. It is basic math. I know for a fact that 6-1/8" back spacing on an 8" wheel will tuck. Is it smart to do that? Nope. Is 6-1/4" better? Yup, but not really worth the 1/8" difference. You really need 6 1/2" on an 8" wheel. That translates to 7" on an 8.5" wheel. Once you get so tall a wheel, or so low a truck (body drop) you have to start adding more for the rear. As impounded said as the rear rolls in and becomes more of an issue than the front. I lay full frame right now on 20x8 Intros with 6-1/8" backspacing. However, the this required some MAJOR trimming in the rear. My plans right now are to narrow the rear 1.5" on each side to allow for the wheels.
Now I could go ahead and interpret into all that sissy FWD offset crap, but I really don't feel like taking that much time. Besides I only do billet. I will give you this. In billet wheels (the example I used) the actual wheel is 1" wider than the stated with. So an 8.5" wheel is actually 9.5" outside lip to outside lip. So measuring 7" backspacing (to the mounting pad) on an 8.5" wheel leaves 2.5" from the mounting surface to the outside of the rim.
Offset on the other hand measures from the center of the wheel in milimeters. There are 25.4 mm in 1 inch.
There you go guys... put your high school math skills to the test. Oh, and have fun!