backspacing 1990 S-10 Blazer

I just bought a 1990 S-10 blazer and I was wondering what it takes to lay 20's all the way around and what backspacing to get? Any help would be awsome thanks.

I've got 20x8'5's all the way around on my S-10 and the BS is 5.5". It's a little much for some people, but it still works. I think the "norm" is 5" on an 8.5" wheel.
Chris