bugpack:
Press In Studs 14Mm X 1.855
Press In Studs 14 Mm X 2.20
The problem with buying studs is that the sellers sometimes don't know how to measure a stud. You almost have to have it in your hand with a ruler to be sure you get the right one. These studs above are most likely measured by their overall length. The correct way the rest of the entire world uses is from the underside of the head. Many of the studs (I think these ones too) have an unthreaded portion at the end. As stated above, if you use original P type Al nuts, you have to cut off that worthless part. Then what's the real length? Nobody knows.
Whatever stud you pick, get on the phone and don't hang up until the guy on the other end has put a ruler on it, measuring from the underside of the head to the extent of the threads. Hopefully one in mm.
Porsches came equipped with studs extending from the rotors by 30mm. You can get away with 2-3mm more. Add the thickness of your rotors where the holes go, there's the length you need. Spacers? Add that.