The rack centering bolt is a M12x1.5 that I ground to a point on the end. Standard pitch on M12 is 1.75 mm, 1.5 mm is fine pitch.

Center bolt for the steering rack is a M12x1.5 (be aware, common M12 has a 1.75 pitch, so this is fine pitch).
The part # 000.721.913.20 (Porsche Tool 9132), cost is $7. They are very handy and I keep one in the glove box of all my 928 so I always have it when alignments are done.

Here is a photo of one if it helps.

So because I hate jumping in the car to go get small parts, and before I got the grinder out, I looked around for an M12x1.5 I could sacrifice. Almost gave up looking, then my old fuel pump check valve called me, and said what about me?. I looked and envisioned, taking a break from tie rod ends and sway bar bushings. The pictures tell the story. Drill it out, put in a Round head screw of sheet metal or wood screw type to make that center point. Done. It works great.