I'm building a reverse flow smoker with what I was told is a 40 gallon tank. According to feldon my 1/2 moon opening should be 11.19". 49.15 sq.inches. the ? I have is the 11.19 at the widest point of the 1/2 moon or is this the total of the opening .or can I just cut the entire opening from the fire box .the box is 24x16x16 setting in the box at 5". Thanks for the help.