As I understand it, backspacing is the distance from the back face of the wheel where it bolts to the hub, to the back edge of the rim. With a straight edge across the diameter of the wheel you can measure from the center of the rim to the straight edge. It is easier to do with the tire dismounted so it is not in the way.
Cage Up, Wheels Down
Jeepfiend
All my Jeeps are in pieces!