Again:

"The way to have your ‘cake and eat it too’ though is to plot the 825/828’s area expansion every ‘x’ distance [of your choosing] and extrapolate it out until it meets your mouth dims to find the correct axial length, flare frequency and closest radius. Now it should perform like an Altec through the XO BW except with a bit more bottom end and directivity control."

Conversely, work out the Sm based on axial length and start with Altec's 128"^2 throat if you want the largest practical one.

GM