If you type in something like 10cu' (or what ever box size you have) and 42Hz (or what ever freq. you want) it will give you a port length. If your trying to figure out the size of the box, just type something in and keep changing it until you get what you want. You can also decide how wide and tall you want the port. What I would do is build a box that's size would give me the largest port area...and have it's peak match up with you RF.
2006 Jeep Liberty
Adire Audio 15" Brahma running off Cadence A7hc, Clarion hu, MB Quart 6.5s and 5.25s running off old school Punch 800a4
146+db @ 38hz with 1200w and a single 15"
Lv = [(14630000 x R^2) / (Fb^2 x Vb)] - 1.463R
*Vb = vol. in cu"
**R = sq.rt. A / Pi (for square/slot port)