From memory the FAA (USA) and CAA (or JAA Europe) had a different certification philosophy with regard to FMC predicted max altitudes and cruise c of g. The Americans used best case c of g (highest alt capability) and 1.2g buffet onset. The Europeans used statistically normal case c of g and 1.3 g. When it became technically possible to automatically update cruise c of g in the FMC the Americans refused, because they lost apparent capability. So for a long time it was only possible to enter the c of g manually into the FMC - that may still be the case, I don't know. Typical in service figures would range from about 11 to about 24 depending on a/c type etc.