Initial calculation
Hello Chris,
My company's MSNs are in between 3000 and 5000ish.
At the beginning of the flight FAC uses crew entered data on MCDU but it changes to FAC's own calculation after switching to aerodynamic calculation model. (#6 says a bit different) At least that's what I know. I found an article as an answer from Airbus on old WISE which states that.
I think this is an important subject and FCOM should be more detailed on matters like this one.