Areo port, port area

Its been a long time since I built my own box, I searched but can't find the thread that had some basic port area specs. 

I am building a areo ported box for a Fi 12Q, 2.5 cubes tuned to 40 htz on a hifonics 2400.1d.

I am thinking a 4in port like 5.5 inchs in length will put me where I wanna be but figured I bounce it off guys that do this more often than I do. overall box specs are 22Lx20Hx14D, sub and port firing up. Gonna run this in my trailblazer.