I just noticed you are using a kicker sub...similar to my build. Im using one L7 15" though with two ports, but I am only using two to make it symmetrical.
Use this to figure up port length if you are doing two. Only difference to get tuning with more than one port is to take your expected airspace and divide it by the number of ports. So if you are going for a 5 ft^3 box, you would put 2.5 ft^3 in this formula instead.
Lv = port length
Av = your port area (height x width of the port face)
Vb = the internal volume of your box
Fb = the tuning frequency you want
You could also use the calculator on carstereo.com but this is more accurate.