You are correct my friend!!!
I´m a fan of trial and error myself, just for example, i was using 17 SQ per cubic feet with my DD 1510´s and sounded awesome, amazing output, yes the Xmax it´s high so i used a big ass port, then, MTX gave me some nice 7500´s , built a box for them with a smaller port area because of the Xmax wich was way smaller, sounded nice and scored a 146.5 dbs, then, i got them to the DD´s box with that huge port and scored a 148.1 dbs , according to people´s experience, myself, calculators and everything they should have sounded better on the smaller port box, but it wasn´t.
My point is, and i might be wrong, but port area also depends on box size and car, my little suzuki swift HATES big areoports as much as it hates big subs, once again, i might be wrong, but after testing tons of boxes and subs in a few years, i prefer to play with port areas until i get the best one instead of using some calculator , but that´s just me...