Symonds, afternoon..... well, In my opinion, the FB air inlet was calculated to provide adequate air flow through the CC to keep it hot... and keep the temp up..... It's difficult to explain.. the volume of the CC and that surface area, require a certain amount of air flow for all the numbers to work.... Btu's, CFM... etc.... No one runs their smokers with the bottom vent fully open.... It's there to start the fire, move a great volume or air to get the smoker up to temp...... Opening the door during start up will help...... Having a second air inlet above the fire grate to move air without having to stoke the fire is important and can play a roll in initial heat up of the unit....
Soooo.... that being said.... go with what you have, make sure the air inlet above the fire grate is 5-6 sq. in..... and open the door, if needed, during warm up of the smoker....
The calculator numbers are there to insure you have a smoker that works... If stuff doesn't work out, and you can't fit your smoker to the numbers, then that's the way the wind blew today... No biggee... If the smoker needs more air, you will find a way to make it happen.... If the smoker runs fine with what you have, the that is really cool.....
Dave