You never ask for an amount of Litres/Gallons/Kgs to be put on board, you always ask for what you want left in the tanks after refuelling. That way it's harder to make fuelling calculation errors. If we want to depart with 50 tonnes, we ask for 50 tonnes. On the B767 there is a 200Kg "wobbly bit". As stated above, 5 mins after fuelling the figure will drop 200Kg, so if you ask for 50 tonnes, you usually see 50,200Kg. If you see 50,000Kg its not worth worrying about.
The idea is to fuel to 2 tonnes less than required, wait for the updated ZFW and then load as much as required for the new weight, in case it has gone down. In practice this rarely happens as keeping the fueller there is not always possible, or the latest weight may come too late. ZFW rarely goes up enough to worry about extra fuel, but if it did you could call the fueller back.