rear horn mouth area

I am currently building a pair of rear horns for some 3.5" speakers i have based on Marting King's mathcad worksheets. My design has a length of 120", throat area of 2.25 in^2 and a mouth area of ~144 in^2. the predicted response is fairly smooth to 50hz. however, i've been reading some other information online and a few sources state that the required mouth area is related to wavelength of the expected cutoff frequency. so is the mathcad model wrong or is the required area just a suggestion? I didn't see anything about this in the papers on horn design that King created.
 
I think what you have designed is a TL that transitions to horn-like behavior. The horn cut-off frequency is determined by the mouth size and your 144 in^2 is small so the horn cut-off frequency is high. Below this point I think you will have TL behavior, if you look at the acoustic impedance plot in the MathCad worksheets you will see a series of peaks at low frequencies. Then at some point the coupling volume will roll off the "mouth's" output as the line becomes more like a horn, look at the open end SPL response plot to see if this occurs. Nothing wrong with this approach, I think most BLH designs on the Internet do exactly the same thing.

My write up on advance BLH design is attempting to show this compromised design. I am updating, cleaning up some typos/explanations and improving the baffle step part of the calculation, and will post a new version this document later in the week.