I generally go a little large on cubic feet, my 15 is in 6ft^3. as for tuning, usually you want something like 13-16 square inches of port per cubic foot of box volume, so my box port is 9x9=81sq. in. and 14in.x6= 84sq. in. so im pretty much right on. this lets it breathe correctly, but tuning is basically port length. the lower you want your box to sound, the longer the port. there are some great calculators on www.the12volt.com i highly reccomend and i use them all the time.