well if you know the volume your going for then you use that volume and hte port area you need. find your port length, use that to find its displacement. (dont forget to account for port walls) and add that amount to your volume when calculating it.
hope that makes since?
like if i wanted 2 cu ft @ 30 Hz with a 2x8.75 port i would input that find my length of 20.65 multiply 20.65x2x8.75 then divide by 1728 giving you .209 cu ft. so when calculating for volume youll look for 2.21
this equationg assumes no port walls. youll want to add the wall thickness onto the width/ height as needed