Offset is what will determine what the backspace is.
For example if you have a wheel 10" wide with 0 offset it will have 5" of backspacing. If you had a 10" wide wheel with +25.4( i know its odd number but its easier to show here) than the wheel would have 6" of backspacing. -25.4 offset would render 4" backspace. Hope that helps. So all you need is the wheel width and the offset to figure out what the backspacing is. Sometimes they do not list the backspace just the offset. So offset and backspace are just two ways to measure. One is not more important than the other.
yeah u got it. i just went back and read ur post better after i posted my novel above and realized i pretty much posted the same thing, just elaborated on the details a little more...