Good afternoon Ron,
I'm just stating on a reverse flow myself, and it's going to be a permanent structure. The inside of both my FB and CC are going to be lined with firebrick. I ran my numbers on the calculator accounting for the firebrick and the square inches that it used up. I would do that as well if I were just putting it in the fire box. Firebrick is 2.5 inches thick and that adds up to a lot of square inches off of your original measurement if you weren't using them. My 2 cents. Good luck!