you can use 24.5" x 24.5" avg from the 21" x 28" as your guide line for tank dia....it might be excat but closer to what you have with 21 x 28.........bobI'm so confused I'm building a return flow smoker, my tube is egg shaped, measures 21" high inside, and 28" wide inside, and gonna be 8 ft long. I'm still not sure about size of firebox, and size of smoke stack, I tried that pitt calc. still not sure if I did it right, any help with this whould be great.