Hi, So last winter i came across a 250 gallon propane tank that i was able to acquire for for $60! the plugs and gages had already been removed and little smell remained. This summer i filled the tank with water and let it sit for a week to make sure it was safe to cut and finally this week I've gotten around to begin working on it. I began with making a cut to accept the firebox, i started using a cut off blade on my angle grinder and then quickly switched over to my OA torch to get it done. the cut is 24 inches across and 8 inches tall. I did this before using the Feldons calculator but from what I'm coming up with now its actually pretty close to what i need it appears. So... i ran the calculator to get my firebox size required and decided im going with 28x28x28 that comes out to 114% so i reran it based on reading peoples comments about how it uses that to figure the chimney and air vents to get the actual specs for that. Based on the attached photos can someone tell me if i did this right? I do live in NW Ohio so i am going to insulate the firebox so it can be used in colder weather without consuming so much wood! Here is the planned firebox size And here is the modified specs based on a 100% size firebox So my chimney with be done with 6" with a length of 39 inches? or am i misunderstanding how this works?