Plus, can be used to reduce internal damping for a bit more mid bass 'liveliness' and fine tune it like a vent to get the best overall in room response.
Yeah, that has been the problem with most of the [known to me] floor models and those that did used the ~0.42 offset with one working at the 0.56 offset I used way back when for 8 ft ceilings.
GM
Yeah, a source of constant confusion.
All TLs regardless of taper have a 1/4 WL fundamental, but depending on taper can be a 1/4 WL [open cylinder/only odd harmonics] or 1/2 WL [open pipe, closed cone/both even and odd] resonator, so an open pipe and closed cone must be acoustically 2x longer to have the same fundamental [F1] as the open cylinder: Resonances of open air columns
As Jordan notes though, the BIB calculator is intentionally tuned an octave below the driver's Fs. This is to both protect it below Fs and get a bit more LF gain BW to better blend to a typical room's LF acoustics.
GM
FWIW, if one 'wades' through the thread there's instructions on how to manipulate the Fs, Vas, Qts' specs to make it any size, tuning you want with the understanding that less net volume [Vb] = less LF gain BW and less driver protection down low: https://www.diyaudio.com/forums/full-range/313921-bigger-bigger-badder-bib-2.html#post5571336
GM
That nice full range diver is probably going to need a very long design. Don't worry too much about he particulars because the driver parameters all have o play together to sound good. I suggest making the cabinet a little big and "mass load" or stuff with fill, cheap fiber glass insulation is fine. That is how I fine tuned mine. Port is big enough to fill after build. Also you can make small slit port at top on one to see if you like I better.I did that and really made a lot of difference. Airy open and not a "boxed in" sound. Good luck. MICHAEL