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.

[retrotech]

I don't mean to burn you out on the list, but I am really confused here. I just mounted a 17x2.25/50 tire on the rear with 1/2" spacer. It seems to fit, but with only a small amount of day light clearance. It is a 55 mm back space, so 1/2" shouldn't be enough. Since I borrowed one tire, and don't have long enough studs I can't drive it, but its in there.
Now I don't know if I even like the look.
Give me some input. I seem to be convinced I want the Falken Azenis tires no matter what. And I thought I liked the 17" 993 wheels.
Opinions?