Perfect BS or Offset for new wheels?

okay Guys, I am going to pull the trigger on some billet intros or racelines. I think I want the wheel to sit where the current Foose RST wheels sit. I believe they are 20x9.5 with a +28 offset. Correct me if I am wrong, but if I order a 20x10 and 20x8.5 combo, I need the the 10" wide wheel to be a +40 (+12 from current for 1/2" gained) and the 8.5 wheel to be a +4 or so (-24 or so from current).

With that being said, that would be a +- 6.5" OS on the 10" and 4.5" on the 8.5" wheel. I want them to almost tuck inside fender and I want room to run a 315 DR on rear without hitting inner fender well. I like the even with fender look. Does anyone have pics of staggered wheels and info on their wheels or any input as to what I order? I don't want to spend 2500-3k on the wrong wheels. Thanks