Now I just asked the refueler today what he does and the interesting thing is and I'm not sure if this is standard but he said if I order 90,000 KG not be loaded but to be the fuel onboard after fueling is complete and say we have 10,000KG onboard already and his SG is 0.8 he converts that to liters and dials up 100,000 liters on the truck and pushes the go button.
I think we all just assumed they push the go button till the A/C says 90,000KGs and then the refueler read off the liters that went through the truck.
Bizarre... I've never heard of any fueller doing this for aircraft with automatic fuelling capabilities. Even when the automatics are faulty, we (engineers) open and close the tank refuel valves manually according to the Kg or Lbs reading on the aircraft refuelling panel gauges. The aircraft have their own densitometers and compensators for computing weight.