I'm assuming by backspacing you mean the wheel offset specification. The U.S. service manual says it's 45mm or 1.8 inches and is the same for both 18" and 20" wheels.
That's a new one on me. Nothing about it mentioned in the service manual. In my entire life when it comes to wheels the only dimensions I've ever seen mentioned are diameter, bolt pattern diameter, width and offset.
I suppose, technically speaking, you could calculate that since you know the wheel width. By your description, it sounds like it would be 1/2 the width +- the offset (depending on which way the offset is - positive or negative).
