How to Measure for the correct back spacing?

Hello everyone. I am ordering 15x8 steel wheels from wheel vintiques. I am not sure if I need to order the 4 or 4 1/2 back spacing? They are going on a 1973 duster. The rear suspension is completely original. I want to put a 255 70 15 tire on as well. Is there a way I can measure for what back spacing will work? Or if you guys already know I would appreciate the help. Thank you