Exhaust Chimney Volume question

[babyboomerboy]

Using the ratio program, for my build, the exhaust pipe lengths for a 4in. wide pipe is 77.63 inches long and a pipe 6in. wide would have a length of 34.50 inches long. My question is about where the length begin. If I start my exhaust down at the bottom grate level, is that where the length begins, or beginning at the top of the smoker? Hope I explained that correctly. Thank you

 

[pineywoods]

The stack on my reverse flow is cut to follow the contour of the tank and doesn't extend into the smoker at all