That's correct, if you design for 7.9 cu ft that is net volume. Your gross volume then needs to be 7.9+0.22+0.343=8.463 cu ft. Make sure you take into account the thickness of the material, if you only take exterior measurements you'll fall a bit short. Are you planning on any sort of bracing? For a box that large on 500+ watts you are probably going to want some internal bracing to minimize box flex. You also need to take into account the volume of any bracing you are using into your calculations.