wheel spacer confusion, 17" 993 wheels Options

[retrotech]

I just got my 17 x 7" 993 wheels. I was told they are a 55 mm back spacing, and I would need 20mm to make them fit
When I measure from the back side its 5 7/8 to rim edge.
I measure my 15" deep six wheel its 4 7/8 to rim edge.
20mm is about 1/2" right? It would appear I need 1" spacer.
Am I missing something? Once again, what size spacer have others used for this aplication?

thanks for your help.

[smrz914]

The idea behind test fitting the wheels is, as I understand it, to figure out what size spacers you need. Mount the wheel/tire on the car. make some wooden wedges that will pretend to be your spacer and they will be adjustable. when you mount the wheel check the clearence on the backside of the wheel/tire and the trailing arm. (I think I had to move my brake line a little so the wheel/tire spun freely. After you have the back spaced correctly then see what needs to be done at the outside of the wheel/tire and the quarter pannel. I think you are puting to much thought into the spacers. Figure out what you need and then find out what sizes you can get. keep us posted on the progress.