Im building a 500 gal. reverse flow smoker, my tank is 48 in. diameter and 60 in. long, Feldons says I need a 36,172 in. cu. in. firebox.. this comes out to roughly a 32x36 in. box, well Ive got the box welded up and this this is giant, its made from 1/4 in. plate and probably weighs 300# or better by itself... does this sound like the right size for this size cooker, Ive did the calculations 5-6 times and its right according to the calculator..are you supposed to deduct the space under the reverse flow plate or is it figured in.. What would be the main disadvantage to a reg. offset smoker vs. the reverse flow? This is my 1st try at building a smoker also. thanks for any help.