FCOM 2.01.40 p3 has a sample manual LDS/W&B diagram that illustrates the process nicely.
1 get ZF index
2 get index correction for tkof fuel
3 combine 1 and 2 to obtain index at takeoff
4 get TOW (GW + TO Fuel Weight)
5 enter chart with TOW and TO index to obtain TO CG.
I suspect though, that you are asking about how FMGS calculates the GW CG at FUEL PRED pg MCDU? The description is at FCOM 4.03.25 p 16. But the princple must be the same.
1) you enter ZFW and ZFCG
2) machine reverse calculates ZF index (not displayed)
3) machnie reads the amount of fuel from sensors
4) machnie enters a stored fuel index table to work out actual index (not displayed)
5) machine figures out GW (displayed) and from a stored formula calculates CG position (displayed) that applies for this weight and index.
6) if you happen to have the exact amout of fuel in tanks as you will have for takeoff then GW CG = TOCG.
Or something like that.
BTW, what use is FMGS calculated TOCG? That's too late to set the trim and besides you need to figure it out before you start loading pax / cargo, to make sure you do not end up with TO CG that is beyond the limits, right?
FD (the un-real)
PS: Of course, "weight" is incorrect term to describe what in fact is "mass" as long as the units are kg/lbs.