Thanks for the follow-up. I've done a lot of reading since posting (Fred Olsen's The Kiln Book, Nils Lou's The Art of Firing , Mel Jacobsen's 21st Century Kilns, and even Kiln Construction by Joe Finch), and have also plugged in numbers from different kilns in the books and those I have access to locally. Unfortunately, there are no consistent ratios here. I love the idea of the double venturi as described by Nils Lou, but after running the numbers on his MTF kiln (that he uses as an example of a double venturi), I found the area increases after the entrance to the exit flue as follows: 31.5