Hi Malcolm,
I built a 3.6 metre diameter 3V geodesic dome using this link for the dimensions:
http://www.desertdomes.com/domecalc.html
I can't recall where I found the joining angle, but I used a nominal 11 degrees. I reckon that trial and error would be the easiest way to get the foam cut angle correct. A smaller diameter dome would need a larger angle between the joining faces, and vice versa.
Chris