I am making a reverse flow 275 gal. tank smoker. I calculated it to have a cc of 206 gal. My cc / fb is 192 cu in. I was wondering if the fire box size is measured from the top of the fire grate? I am making the fire box 30.5 talk 28 wide by 28 deep. Is the height measured from the fire grate? Are my calculations going to work?