The way I do it, is to take the tire off and then, with the jack under the axle, take a 24" carpenter square and butt it against the axle flange. Measure the distance, from the edge of the square, to all areas that encompass the tire, keeping in mind the actual section width of the sidewall.
I would subtract at least 1/2" from that measurement for the back spacing using the sidewall as the reference point.