Math will not a good smoker make. Look at it this way, do you want 1 or2 racks? If it's 1 then picture what you will be putting on it. Same thing for 2 racks. That will be the size of the door. If you're doing bacon and brisket then a 20" door does not make sense. If you're doing whole pigs then the 20" does make sense. You can also make a second special purpose one.
