I'm assuming he has an Indexing head and a mill. makes this super easy.
by the way I'm jealous
I wish I had it that easy.
This was all done with a basic drill press and a machinist square like this one:
I marked the location of all the vent ribs with a sharpie on the edge of the rotor face. Using just the "V" attachment on the square I could then draw lines across the face of the rotor with a sharpie showing the exact location of each vent rib. Then I measured to find the exact middle between each rib and scribed a line across the rotor face in between every rib using the "V" attachment again. Then I measured the different diameters of where I wanted my holes drilled (3 concentric circles) and made a punch mark on the rotor face for each diameter. I took that down to a friend who has a brake lathe and he scored perfect circles on the face using my punch marks as the starting point. Then I simply made punch marks where I wanted the holes which was a pattern based off of the 3 concentric circles and the scored lines between each rib. Then drill to size and turn the rotors down on the lathe again. I need to go back and add a small chamfer to each hole, but that will only take a few minutes.