You could probably answer it better than we could. What car are they going on? how much brake clearance do you need so they don't hit the calipers? What size is the pad diameter on the wheels? Have you measured any of this?
I would say around 160mm would be about right.
If i knew the wheel pad dia. I wouldn't need the dia. of the the adapters. lol 160mm sounds about right though. If got clearance, offset all that stuff down. Was only missing the total dia. I don't have any measuring tool that fits inside the mag and is able to extend that much to measure.
It would be 30mm from the center of the stud or bolt hole of the porsche pcd to the edge of the adapter. Depends what way you're going to mount the wheel. But either way it's far from impossible.
What's the clearance from your brake caliper to the center of your rotor/axel?
Ah, well, I guess people are doing that then.
Did some looking, apparently 42 Draft designs does 6.375" with a back cut. See the very end of the page on this link, cool pic detailing brake clearance. http://www.42draftdesigns.com/tech/adaptorfaq.html#130
Other sources have anywhere up to 175mm OD, but not necessarily for 5x100 fitment.