(Box building) How to determine sealed cu. ft. from recommended ported box spec.

[Maurice D.]

I have purchased the software Term-Pro to design a subwoofer enclosure for my 1997 Ram. I have been learning to utilize this program effectively, despite being a novice. Currently, I am attempting to create a tri-chamber 4th order enclosure for two American Bass XD12D4 subwoofers in my 2nd gen. However, Im needing assistance in determining the sealed cubic feet requirement from the recommended ported box cubic feet specification of 1.75 cubic feet. Can I use this value for the outer chambers, or should it be adjusted?