Hey guys I'm from New Zealand and we don't have 250 gallon tanks available here. I have built my own from 2 upright tanks, cut one end off each and welded them together for the cook chamber. I have made the firebox from some 6.125mm (or 1/4 inch) steel and I am abut ready to put them together. I am struggling with the throat size, between the firebox and cook chamber. I have put my numbers through Feldons, and the smokerbuilder calculator (all results identical through both) and it has said I need a 142.5 square inch opening from firebox to cook chamber. I have tried to use the circle calculator but cant figure it out. Can anyone tell me what size the half moon opening should be? The fire box is 25 inch wide so the opening can be upto that length, I just don't know how deep, or what shape. (i.e is it a half circle or is it the bottom of a circle if you know what I mean) Here is the link to Feldons - Link to BBQ Pit Calculator One more question, what is the effect of over sizing this opening? Being to small would be no good but could it be bigger? Over size firebox and exhausts are used regularly, do people oversize the firebox to cc opening too? I will try and upload some pics soon. Thanks in advance.