I am ready to order my thermal plate but need to know if my firebox to smoke chamber dimensions are ok. I have done the Pit calc. but can not figure out the cubic inches needed on a circular shape (moon shape). The fire box is 1/4' plate steel and the measurement from the bottom of the smoke chamber to the top of the firebox is exactly 5-1/4", so that gives me 5" when the firebox is cut. Thatis exactly 1/4 of the tank dimensions (the tank is 20" in diameter). Is there a mathematical way to figure out the odd shape? Pics to help The bottom piece of wood ios where the thermal plate will go and the above piece is the cooking grid level, it will be slightly higher about 4-1/2" from the thermal plate.