ET 20
Is that the offset of 20mm? If so is it positive or negative?
You need to know the width of the lip of the wheel. Offset is measured from the centerline of the wheel as determined by the inner and outer tire mounting surface. The backspace is the measurement from the mount surface to the inner most part of the wheel lip. Just to make sure I'm making sense, by the width of the wheel lip, I mean the distance from the tire mounting surface to the outside of the wheel's lip. I usually assume this at .5", but depending on the wheel it could vary.
Using your numbers and assuming that the offset is positive (more dished inward) here is what I come up with.
9.5"/2 + (20mm/25.4"/mm) + .5" = 6.03" Backspace
Here is a good diagram I found on TireRack
