I am in the early stages of a new RF trailer build using a 6' by 24" chamber 1/4" thick. I have downloaded the calculator for my flows thru the smoker, the question I have, when figuring the chimney diameter do you use the ID, or OD measurement. There is a HUGE difference when using 4" and 5" pipe becuase the inside diameter is 3.5" or 4.5"(respectively). For the 4" pipe, the OD would be 43" tall, while using the ID the number come out to be 55". As soon as I finished the plans I will start a new post showing progress, BTW, thank-you to everyone that has already shared their smokers in progress, this is an AWESOME site for do-it-yourself builders and I am glad to share what I have learned along side with some new ideas I am going to try in my build. *NOTE, I will be using 1/4" pipe for the stack.