the reason why they change, is because as you're increasing port volume, you're decreasing the net volume of the box, which will slightly change the port range. it's not necessarily supposed to be relied on 100%, i just got tired of having to figure port area out every time i played around with a box design and was like hell...i can throw that on the calculator so i did. just as a guidance more or less.
if you do a slot port, which is what i'd suggest if you're trying to get lower tuning without sacrificing too much volume, then you can get a slightly lower tuning frequency with the end correction of the port. although...with the port volume, i dont think you'll be able to get anywhere above 2 cubic feet per chamber.
the best i can come up with is the same as krannymans box. tho i dont think i'd do it 2" x 20". that's a pretty thin port and could cause port noise. the general ratio is 1:6 or 1:7 (cant quite remember ) and that'd be 1:10. might work, but might not. 20 sq inches of port tho will suffice. and get you very close to your desired port tuning
oh 12-16 sq inches is also what i use for ports as well