Generally speaking you want to select the wheels with the offset you like first. Then mock them up under the car to determin the WMS to WMS width of the rear end. Here is a good post on exactly what the WMS measurement is and how to measure it. Just for the record, my rear wheels are 18x12 with a 5.9" back space. I mocked them up under the car where I wanted them and determined that I needed a 57" WMS to WMS rear end.
http://www.pro-touring.com/showthrea...Mount-Surface)