No one statistic tunes frequency. All aspects of a box have some correlation as to the final tuning. True, making a port longer lowers the frequency, but also making the port a different size, such as cross sectional area, and modifying the box's internal volume also changes the tuning as well. I suggest just using a box calculator, they are accurate and easy to use. Just remember, you want to have enough volume as recommended by the manufacturer with the port volume subtracted. Then you make the port large enough area wise so that you don't get port noise from turbulence. Then you make the port long enough to get the right tuning. Keep in mind while making the port larger and longer you are shrinking the box's volume that the subs have, so it's basically a juggling act of tweaking each and every aspect of the box till each spec is where you want it. Then start mapping out a build plan and go at it.
Good luck //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif