Setting the draft on a 2x6

[nymapleguy607]

Do any of you have a 2x6 evaporator with an 8" stack. I'm trying to find what the spacing if from the bottom of the flue to the stack collar as well as spacing from the stack collar to where rear taper. I've boiled the last few days and whenever I boil the internal stack temp runs up to 1300-1400 degrees, when I fire it the flame is being pulled back through the flues really fast. I know Leader says 2x the length of your pans but thats using 10in pipe. I have about 20ft of stack but if you calculate the stack volumes the 2 are within 12 cubic inches of each other.

[Flat Lander Sugaring]

my original arch from leader came with a 7" stack and on natural draft at times I would have flames out stack. When I added AUF I would get flames out stack every time which looks cool but not efficient.
when i built my arch this summer, what ever the spacing was from the top rail to floor of arch under the flue which was approx 7", I kept that same distance to the rear of the flue before entering the stack area.
Hope this helps