I agree that technically it is difficult to develop. But the actual margin it is not safe at all. I have noticed on some airports, always the same ones, that I always burn a minimum of 1% more than predicted for the flight. One or two tonns is too much when you plan to land with 8 tonns. I suspect that they do not stop excessive hand baggage and they do accept more than authorized luggage weight. Heavy luggage is usual at vacational destinies. The actual standard ICAO weight is insufficient for the big trolleys nowadays. Small errors on each of 300 pax becomes a big error.