A quick note, the area under the RF plate does not need to be larger than the firebox to chamber opening, it is suggested, but not necessary...my RF plate ties in to the top of my firebox which enters the tank at 1/3 of the height of the tank and about 6 inches into the width of the tank. I can cook at 200 or 350, you name it, clean fire, thin blue smoke and no drafting problems at all. However, the biggest item of concern in building a RF smoker is to make sure that the opening at the end of the RF plate is at least as big, but preferably bigger than the opening from the firebox to cook chamber. This will insure that you won't choke down the air flow and create any draft or air flow problems with the rig. Of course making sure you have the properly sized exhaust and intakes assist greatly with air flow as well. Another note on RF plate placement, it should be at least 4" below the bottom grate of your pit, if you can go more and still get the calculations worked out, do, but I wouldn't go anymore than 6". Good luck on your build and we can't wait to see pics!