vent length

hey i'm building a custom center console with a sub box attatched to it for my truck, i have an alpine 10" type x and want a vented box, on their website (www.alpine.com) and manual they show what internal volume and vent length and volume should be, the only thing is that my box doesn't have a side that i can put an 11x1" vent, i want to put a 15x1" vent, i tried using the port length calculator on this website, but i don't know if it works the same for a rectangular vent because it is for a circular port. i've also read that a bigger area port requires more length for the same effect. if this is true than why wouldn't people just put a 1" port so that the box can be a lot smaller and so can the length of the port.