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]

I'm not going to debate the spacers here but we can start a new thread.

retrotech, If you like I can put one of my front rims on the back of my car to see how it works with the 7" rim on the back. It'll have to be after work tomorrow though.

I spent a whole day figuring out what size spacers I needed for my rims. It took the time and did it only once.