Ive read the 12-16 square inches of port per cubic foot of box. The problem im having is getting a port to work out right with the tuning and size. The box size I want to go with is 42"d 48"w 23"t. I want the port to be on the 48" rear wall. My amp will be below the port so I dont want to do a bottom slot port. Im trying to tune around 32hz. Every port that I try gives me the right tuning but not enough port volume.