the difference in the calculators is for slot port correction. the straight slot ports that were done in marty subs needed some correction factor too or they tuned too low.
not's build has a 90 degree bend, which has the effect of increases the port length because it doesn't flow as well as a straight port (at least that's my theory).
this size cab is also effectively the same as two full size front firing marty's (i'm not sure if that makes it a 'double-wide' or just a 'fat marty' :-)), so all the drivers that would work in that one will work in this one...stereo integrity ht18, uxl, 460ho, etc. as that question always seems to get asked.
also, there are a number of 15" drivers that would work in this enclosure as a quad set.
I like the approach to form factor as it keeps the cuts to a minimum (heck, home depot might even be able to make most of the cuts for you). having a simple plan, picture, and cut sheet for folks who don't want to try to design from scratch is awesome.
nice work 'not.