I recently measured for back spacing my self. I determined that we have 1" more clearance in the back than the front. first you have to decide what width wheel you want to run, devide the width by 2 then add 1/2" examples:
7" rim; 7 / 2 = 3.5 + .5= 4
8" rim; 8/2 = 4.0 + .5= 4.5
