yeah, calculating the actual box volume isn't hard with just MDF...its next to impossible to be exactally accurate with fiberglass...theres even a calculator out there for finding the airspace of a box, even factors in the thickness of the wood and everything...also, depending on how the box it built, do u see any way to slightly modify it to get 2" extra out of it for the sub? like is there atleast an inch of material on each side that you could cut out for the 12s...BTW, why can't you stick with 10s?? sure theres not as much area, but unless you are gonna compete in sound comps or something, 2 10s will be plenty enough bass if you build the right enclousure and have the right amp powering them and everything...