http://gs.tolan-hoechst.com/tirecalc.htm
lots of cool things like this out there on the internet, anyway offset is just another way to calculate backspacing except the offset is measured from the center of the wheel instead of from the inner lip to the wheel hub. Would be best to measure on your car with the suspension you have for how much backspace you actually have (take a ruler and square it with the face of the hub and measure to the suspension members).