If you were referring to the length of your stack at the end of your question, I would measure from the center of your 90 (your "take-off"). Example: my calculations called for a 14" long exhaust using 4" diameter pipe. I also used a 4" 90 coming off the cook chamber. The take off on a 4" 90 is 6". Then I added an 8" length of pipe to that.