The mathematical angles actually vary according to the frequency and according to whether it is a 4, 5, or 6-star hub.
The tops of the hubs are flat, but each arm in the megahubs tapers off at 12 degrees. In the standard hubs, they are curved at 10 degrees.
Using the 3V megadome as an example, there are 15 hubs around the base, forming a circle. 15 hubs X 2 arms X 12 degrees per arm = 360 degrees. Exactly what you’d expect for a circle.
But there’s fewer degrees on a 2V and many more degrees on a 4V, 5V or 6V. So how does that work?
The answer lies in the flexibility of the PVC. If you make a dome out of rigid steel, there is no tolerance to play with. When you use PVC pipe, there is a great deal of tolerance, and the angles provided in the hubs provide plenty of curvature to start the turns. 10 or 12 degrees is actually just a mid-angle number that works well in the broadest range of applications.