They're all giving me different answers for tuning, how do I do the math myself?

Box is 7 cubes after displacements - 20h x 38w x 27.22d with 1" MDF and the port is inbetween the subs. Do I minus the port width from the length of the box minus the MDF width? For instance 27.22 - (port width 5.75+1" wood) = 27.22-6.75 = 20.47 actual port length, or is it 26.22?

For 20.47 port length:

RE says 31.5hz
The12volt says 37.165hz
Mobileformationlabs says 35.43hz

 

These calculators don't all account for port end corrections & some use different end correction values. Depending on how close the enclosure port(s) are to other objects may change tuning also.

I've found TermPAK & BB6P to be quite accurate.

 

I would have guessed RE's calculator to be quite accurate as well, because they tell you how to cut all your pieces as well. 12 volt for me gives about the same numbers as RE, they just round off differently so there is a slight discrepency, but nothing I would consider to be significant.

I think the problem you're running into is taking the port into consideration. RE uses outer dimensions but the volume is minus port volume. I just did a box using RE (43w x 20hx 18d, port height is 18, 4.5 wide with 4.5 inside piece, comes out to 24.75 total length tuned to 34.6003459849206 hz) and then put the numbers (volume, port size and every decimal of the tuning freq) into 12volt and the difference, my guess is because of rounding, was .1 to 24.76xxxxxx port length.

 

It's for 2 BTL 15's in 7 cubes after displacements with 103.5 (14.78 per) port area. The only difference is the port will be between the subs.

Width - 38
Height - 20
Depth 1 & 2 - 26.51
1" material
5.75" port width
0" port length 2
7.42 cf3 internal
31.53 hz

It doesn't take into account the displacement, so at 7 cubes the tuning would be (I think) a little higher.