Correct backspace

[Outlaw87]

I've been looking but having no luck with finding post on getting the proper backspacing on new wheels. My truck is an 87 Silverado with a Belltech 4-6 drop. The truck is completely stock right now, just the static drop. I'm looking on ordering a set of 20" Budniks, but I don't know what the proper backspacing or most common would be. I see that 20x8.5 & 20x10 seem to be the most common widths. What do you guys with 20's or bigger have for backspacing? Any information would be greatly appreciated. Thanks in advance.