Phil, I would be inclined to think that you are half right on this one as the optimal diffuser's angle is dependant on the diffuser's lenght. Hucho's book showed 2 diffusers of different lenghts. For the longer one the optimal angle was around 2.5 degrees, while for the shorter one it was closer to 3.5 degrees.
Fabrio posted an image from Hucho's work on this
here