I said as much in post #2.Beaming = speed of sound / driver effective diameter / 2
effective diameter = half surround to half surround
divide by 2 to get half wave
some will divide by 4 but 2 is close enough and makes more sense when talking about CTC spacing.
I think the OP is just starting out and is looking for someone to agree with him. So far, I haven't seen any support for the (overly complicated) formula he's stuck on. Danny Richie has been at it long enough that he has a grasp of the fundamentals.
I suspect this is yet another case of RTFM.
"Yea, brethren let us turn to page 83, table 6.1 in our hymnal..."
https://meniscusaudio.com/product/loudspeaker-design-cookbook/