Omlette, anyone?
Don't forget you're not starting from a null value, during the pre-flight one enters ZWF/ZFCG, the FCMC uses the fuel load and the input value to spit out an initial value for the FG/FM to work with.
Once you've got a starting value then the boxes can apply their magic to get a value out of what the aircraft is doing aerodynamically, as described above.