Hi Erik,
The throw arm is only a handle - it doesn't determine the radius. The radius is determined by the shape of the end templates. Look at about 28-32 seconds into the video on the top right.
Here are a couple of quick and nasty diagrams which might explain it better - or might not.
Basically, just add the distance "x" evenly around whatever shape you want for each end and that will be the final shape of your neck or fretboard.
If one end template has a radius of 7.5"+x, and the other has say 20"+x, you will get a compound radius. As long as the distance from the edge of the template to the face of the support board is the same on both sides, you'll get even thickness of board along its length.
With correctly designed end templates as it is used in the video for neck profiles, this jig will even do the neck taper, though I imagine very difficult to do fretboard taper.
In my diagrams I have more than 1/2 the circle, but you only need as much of the circle as needed for the fretboard. (If my diagram was to scale, the fretboard would be about 8" wide - the diagram was more for the concept - I did say it was quick and nasty)