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]

All I have ever heard from the 911 camp is, no way bolt on spacers for track use. Personally I don't see anyway they would fail. You actually would have 2 sets of shorter studs. Is there any thing I am missing about bolt on spacers. I happen to have a pair of bolt on 1.5".
I see why with flares you would need the 1.5", but with stock fenders that seems too much.
My next problem is I want Falkens, but they only make 2.25x17. Looking under the car now in rear, with 1.95 I see less than an inch of space. It seems a 2.05 will be close.
I can't go buy one tire to see if it will fit.