Your load controller, or equivalent should have given you a ZFW and ZF Cof G, (expressed as a MAC) on the loadsheet. Once inserted, the FM merely adds the fuel index and weight(it knows where the fuel is from the sensors,) and produces a TOW and MAC for TO.
Merely crosscheck this against the Takeoff Weight and MAC loadsheet figures, and provided they are within a nominal limit (usually 2% for the bigger airbuses) this will give you a stab trim for takeoff.
This can all be done within a few minutes of startup. ( unless you have to do it yourself, but even then a laden index ZFW will be easy enough to calculate, using the FCOM tables)
The big catch is.... if the entered ZFW data is in error... the aircraft, and your good self... will be unaware until the aircraft tries to fly. Nasty if it's way out!
Once airborne, it WILL attempt to cross check itself by using a backup CofG computation. It applies a little reverse logic by checking STAB angle, thrust setting, IAS, Angle of Attack etc, to work out if the whole picture is valid. And if it isn't, the bells and whistles start.
Garbage in... garbage out.
Hope this clarifies things a bit.