I need some help, for the life of me I cant wrap my head around figuring out the size of the Firebox-to-Cook Chamber Opening, I have read the tutorial, and it made my brain hurt, I have found the Circle calculator and am not sure which measurements to enter, can someone help me with this? I am using a 40 pound propane tank, the internet says it holds 9.4-9.5 gallons, I figured out there are 231 cubic inches in a gallon. the feldon calculator is right on that, My fire box is 10x10 but I cant figure out the Firebox-to-Cook Chamber Opening and on top of that I have read the size needs to a bit larger than the calculation doe to friction and using a smaller CC. should the opening on the other end of the baffle be about the same size? thanks in advance.