hi all,been awhile since last posting,but my brother and I finally got a start on our reverse flow smoker,i have one problem maybe someone has a simple way to calculate,what is the best or easiest way to measure the firebox opening to cooking chamber,i know its suppose to be 16 inches according to the calculator,but whats the easiest way to measure,im at 100% on my firebox size,i cant afford a mistake here,any help would be appreciated,its gonna be a half moon shape,26" pipe for both the cooking and firebox,right now I know its too big of an opening,would a paper templet work?thanks