Call Vulcraft and they will send you a derivation and a design example. I don't have mine here at the office or I would scan it for you. They will also help you with the calculation.
Never have liked this approach in that only through deflection of the diaphragm can the torsional stresses be transferred to the two parallel walls. This means more deflection or a much stiffer diaphragm, still with more deflection than a four sided model. Professionally, I will not use it, even if it is accepted.
