fi q porting options

[t-rust 0]

im about to switch to port tubes instead of slot ports and i was wondering if i have 2 18" fi q's how many 6" diameter ports would i need. i have the box set up into two chambers at 7.1 cubes each. i was wanting to tune the box around 31-33 hz. i didn't know if i should use 2 ports per chamber or just one and what the exact lengthe should be.....ive used most calculators and they are telling me with 1 6" port in a 7.1 cube box the length should only have to be 6 inches, which doesnt seem right b/c i was thinkin more like 15-16 inches. shoot me some info thanks