Is there any formula for calculating the actual cook door size on a smoker build.. I was given a old fuel tank thats been sitting for 35 yrs. and just now got it into my garage to start on it.. Ive never built a smoker so pretty much new to it all.. the dimensions on the tank are 59.75 in. long x 48 in. diameter. Also not sure yet if I want to do a reverse flow or just a offset smoker.. I went to Feldons and put in all the specs to get everything I need just not sure about the door.. thanks for any help..