It looks to me like the diameter is 299mm (11.77"). Perhaps Alan can confirm that. He doesn't have anything to do except mope around in the rain anyway.
The drawing you have read the measurements from is for the earlier Magic Pies. Although the spacing is different, the PCD
(Pitch Circle Diameter) of the spoke holes should still be the same for the MPIII.
I tried to accurately measure my Original Magic Pie when I had to design and fabricate a
flange ring modification, and according to my CAD files, I've used a measurement of 297.6mm for the spoke hole PCD, but it's quite possible that the 299mm shown on the pdf drawing is the correct measurement.
I tried several spokes calculators.
But the MP III hub dimensions are very different compare to a nomal MTB rim.
But even with an exact measurement, the typical spoke length calculator programs are not likely to calculate the correct spoke length accurately for the MPIII because of its unconventional spacing of the spoke holes around the hub flanges:
Alan P.S. I haven't been moping around in the rain, I've just spent the weekend driving ~900 miles
(up to Scotland and back) to visit my granddaughter for her 2nd birthday, and it rained during most of the journey back home.
