Let me ask you this how critical it the wall thickness for the stack I know the RF plate and CC side need the thermal mass of thicker wall but would say 0.125 wall be fine for the stack
You aren't over thinking things, see the above calculator and the resultant numbers. Things will be different if you don't butt the two pipe pieces together end to end but rather mount the firebox perpendicular to the CC.Ok so I have a bad habit of over thinking things so please excuse me if this is a stupid question but if I make A to B 17 1/2" wouldn't that make C a lot more than 3 1/2" or will making it an arc add enough length to reach 17 1/2"