astro offset/backspacing

I got a 96 astro with a 3in drop all around(spindles,coils and flip kit). I want to run 20x8.5in wheel but don't know what offset and backspacing to get. I need the wheels to tuck and have decent clearance and turning room.

5.5" of backspacing will give you lots of room.

DO NOT GET A PLUS 11 OFF SET

2lowastro is right. get atleast a +20 offset because a 10 or 11 offset will not tuck nicely if you wanna lay it flat