Ported box for 2 15 Obsidian

[mmets]

Hey guys just want to make sure I'm doing this right......which is a toss up. Planning on building a box that will be 7.8 cubit feet but after some bracing will end up at just a little over 7.5 which for those subs is correct. my issue is the port. looking at 12volt.com or whatever the site is called has my port at 7x7x11.4 tuned at 32 hertz. box size is good along with the tuned hertz according to obsidian. my question is the port itself. is this correct or am i missing something. not sure if the port size in the center of the box needs to be taken out of the total cubit feet inside or do i just need to make sure the cubit feet in the box is right and the port size area doesnt mess it up or matter. I want to say thanks in advance for any help in this matter. thanks.