is there a reason why you wouldn't use the traditional equations?
what value would I use for S?
for 2d and 3d calcs
the frontal area of the diffuser, just the part that looks like this ( ?
and would it be the 2d area? or the 3d area?
Follows this refernce http://highered.mcgraw-hill.com/sites/dl/free/0072402172/62802/chVIIb.pdf
Remember the only difference in 2d is F/l so account for that and you are fine
Would highly recommend you start with something with a known answer so you know your method is good