Before I get too much flack about my phase primmer earlier... gotta love PMs.
People seem to be misunderstanding the application here, which means I didn't explain it well enough.
The info is in post 12, 13, and the often
This is the quick/simple way
to use the HR phase response graph to minimize phase shift.
The key mess up seems to be the use of the term "passband" instead of "total system usable bandwidth." I am sorry.
The thing is, you have to figure out the quarter wavelength of your horn, and this is the low limit. The upper limit is how high the driver could
be used in the horn. IE, if the Fr doesn't begin to roll off until ~200Hz use 200Hz. When in doubt 120-150Hz will work.
If you use lilmike's posted HR inputs from post 42 you get a 414cm horn. This gives you an ~21Hz QW length. The FR graph seems to roll of ~200Hz, so you need to focus on the phase difference from ~21Hz-~200Hz. This will show around a 120deg phase difference total from peaks and dips. This lines up pretty well with the measured and AkAbak simulations. This isn't super exact, but it seems to always hold up.
Hope that clears it up a little.