backspacing on 20 inch rim for S10

Im ordering 20's for the back of my S-10 and i want to go decently wide on them. i was thinking 20x9.5 but i need to know what is the common backspacing to give it a deep look but i can still tuck when i lay it out with atleast an inch of clearence. take a look at my profile to get a better idea if you need to. i have 18x8s on teh back in the pictures. also i need an idea of waht tire size would be best.