ok, lets put some numbers out there for the two presumeably most popular choices; SEOS-15-9050-N4 & SEOS-15-9060-N3.
The height of the SEOS-15-9060-N3 is 8" according to jack. Let's assume we use a 12" woofer and a 0.5" space between waveguide and woofer. That means CTC distance is 10.5". That corresponds to roughly 1250hz. So by Linkwitz's rule, in order to have a 60º vertical lobe the waveguide has to load down to 1250hz. If you use a 15" woofer you need the waveguide to load down to 850hz (impractical probably).
The SEOS-15-9050-N4 is roughly 6" tall (although according to Jack's drawing the above one is only 7.2" tall, not 8" for a comparable figure). This means a CTC of 9.5" and needed loading to ~1400hz. Though, since the SEOS-15-9060-N3 is larger it will likely load down further, a sort of balancing act between CTC and directivity control.
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