How reliable is the Airbus FAC calculated gross-weight? I've noticed some variation between loadsheet weights and FAC (Flight Augmentation Computer) calculated weight. That's of course normal, since loadsheet is calculated with standard weights and FAC calculates the weight from alpha and speed (at least as far as I know).  

Question: Does anyone have any information about the correctness and reliability of the FAC calculated gross-weight?  

I'm also interested in how exactly, and with what parameters, the FAC calculates it.
so after having done some more research this is what I have found.The A320 has two independent gross weight calculations. When the aircraft is on the ground the FMS does the GW calculation. Prior to engine start, the current GW and CG are not defined.; predictions are computed based on the pilot entered ZFW , ZFWCG, blockfuel and the FM calculates the TOW. The GW is calculated  by adding FOB to the pilot entered ZFW (GW=ZFW+FOB)  However I don't know what this iteration rate would be.
When the A320 is airborne, the FACs independently determine the aircraft gross weight (GW) by using the aircraft angle of attack sensors. If a significant difference occurs between the FAC and FMS gross weighs (delta > 7 tons) , the  message "CHECK GW" will be displayed on the scratchpad.