My understanding is that at a given temperature and pressure
The beer can only absorb the specified volume of CO2 even if you leave it at that pressure for longer !
I personally achieve the volume required during fermentation.
Once fermentation and carbonation is complete I cold crash and then keg.
Once Kegged it goes into my
kegerator then set the regulator to serving pressure. Hence I don't use a very much CO2.
I assume you have watched Dr Hans video about the calculator.
Once you've used it in practice I'm sure it will make sense. Good luck mate