On my topic, there is everything to calculate and get the idea in theory
Simple mathematics only.
Stock wheel is 4,5" et34 and the usual empi 8-spoke is 5,5" et30, which fits well with stock fenders.
I dont know how much late 944 arms widen the rear, but this can be found somewhere here at germanlook. I think they widen 74mm total=37mm per side;
http://www.germanlook.net/forums/showthread.php?t=1993
http://www.1010tires.com/Tools/Wheel-Offset-Calculator
This is calculated with stock arms, you must add difference between with stock and late 944 arm width and possibly a spacer width.
Adding 37mm it gives these results:
I would also put 30-40mm thick spacer. No doubt the 11cm is best option to rear.