I'll always remember that incredible flight where the Chief Flight Engineer (that mthrfckr's supposed to show the good example, right ?) did two loadsheets: one for the records and the CAA, and another one to know the approximate overload and trim setting.
Anyway...
Under JAR-OPS rules, the standard weight of the bags depends on the type of flight.
-Domestic: 11 kg
-Within the UE: 13 kg
-Intecontinental: 15 kg
-All other: 13 kg
If an operator does not want to follow these standard masses, he has to advise the CAA of his reasons and gain its approval in advance. He must also submit for approval a detailed weighing survey plan and apply the statistical analysis method given in the JAR-OPS Appendix & IEM 1.620 where they throw formulas all over the place to estimate the pax & baggage weight. If the revised standard masses exceed those given in the JAR-OPS, the higher values must be used.
And if on any flight the standard masses (pax and/or bags) are expected to be exceeded, the operator must determine the actual mass.
For info, the weight of a male pax varies from 83 to 104 kg depending on the type of flight (JAR-OPS).
Things aren't always easy unfortunately.