Hey Chris, as standard Porsche used 52mm or 55mm studs for those rims, but that's on an alloy hub which is probably thicker than the mounting surface of a beetle disc.
To figure it out, you want at least the depth of the mounting face of the rim to the bottom of the ball seat, plus the height of the nut, plus the thickness of your hub where the stud pushes through, plus the thickness of any spacers you want to run.
I'll likely have the front discs off my car next weekend, so will be able to tell you if no-one's come up with a definite answer by then...
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