I wondered about doing the plenum all the way down to the top of the firebox like jerry mentioned but wasn't sure if there were any drawbacks to having that large of an open space inside to the chimney. Jerry do you remember where you saw it mentioned and has any one seen one done that way?
Jerry, afternoon..... all the drawings are generic and not to scale..... If you ask how high the first cooking rack should be, you will probably get several different answers.... the plenum size and height, same thing.... some folks think the stack should come out the top and not be inserted into the CC.... others think the stack should be inserted to 6" above the lowest cooking grate.... I came up with the plenum to "sort of fit" the two ideas.... I've seen CC's with 2 stacks... one inserted 4" and the other 4" above the lowest cooking grate...
As far as I know, there is no perfect smoker or perfect smoker parts... the entire design is very flexible with, somewhere close to the numbers in the calculating tutorial, makes a great smoker....
Sorry it's not more specific....
Dave