I honestly don't see how you base port area mainly on box size. If you have a 18" sub with way more xmax, then its going to need quite a bit larger port area than a lower xmax sub. Im not trying to argue, but that's what I have been taught, and if I'm wrong I wont mind admitting it at all. But can anyone give proof? Or point me in the direction of some literature to read or something?
I use carstereo.com when figuring my port area. I put a link below. All they ask for is sub size, quantity of subs, tuning frequency and xmax. No box volume (until you try figure port length)
Car Audio - PORT Size Calculations and Formulas for WOOFER and Subwoofer BOXES