Backspacing/offset for bd s10

I understand the terms backspacing and offset but get confused on combining the two to determine a total. Im about to order some 20x8.5 billets with 245/35/20 tires and i want as much lip as possible, but will still be able to tuck after i bd my 98 S10. Im thinking around 5" backspacing but not exactly sure. Thanks alot