bruno, morning.... The folks who put this calculator together (Alien), did a lot of research and calculating, to provide a good tool to get the build correct...
If you have concerns as to whether or not it will work for your build, the firebox to cooking chamber size is one I would use.....
As far as openings, you could increase the firebox/cooking chamber opening by 10% and the same for the exhaust stack numbers....
That being said, if you build the smoker per the calculator and leave room for the FB/CC opening to be enlarged I would do that....
The exhaust stack dimensions... you could always add length to the stack to increase it's dimensions at a later date...
Inlet to the firebox, you could add 10% to that also, or leave room to enlarge the holes at a later date....
Trying to build a perfect smoker on the first go around is pretty close to impossible... This calculator will give you a very good start...
I think most folks here have altered their smokers at least once to make improvements to their liking....
I notice the opening dimension at the end of the reverse flow plate is not given....
Solaryellow has given that dimension on his build post....
Since the smoke/air/heat has to take a 180deg turn at the end of the reverse flow plate, I would make it at least 2 x's the FB/CC opening size... and leave room to make it larger.... Just a guess.... no facts....
I would start with the calculator numbers and leave room to adjust the 3 dimensions at a later date, if needed....
Dave