I've searched but can't find exactly what I'm looking for. I'm starting a reverse flow pit. I was given a 4' pipe which is 20" diameter and 3/8" thick steel. I'm wanting to use this pipe for both the pit and firebox. So what I'm trying to figure out what size to make the firebox. If I make a 36" pit that leaves 12" of a firebox which the Feldon calculator says is correct but that seems too small to use. Basically I have 48" to divide between the pit and the firebox. What would you suggest?