BL 15 Sub Box Port question.

[Leon Tan 2]

Im making a box myself but im having a little dilemma im my box construction... i have a 4.35 cube box... im plaing to make the port 2.75 inches wide.... im just stuck on the port length. iv been using diffrent websites and calculaters, but i they all say diffrent port lenghts... iv used 12volts.com, re calulater, torres subwoofers calculator... im thinking the trres calculater is good, but im just geting a second opinion... can somone help me out with this dilemma im having? the specs are 16 height, 35 wide, 17 depth, 2.75 port width, Length? 21-28 inches... wanting to tune it to 31hz.