VonLuffers, you said:
...the FMC will assume standard temperature making it -61 at FL 380
ISA temperature lapse is down to -56 degrees at 35000 ft and thereafter a constant -56 degrees in the Stratosphere......(approx).
Nigel,
Of interest, one of the pin selectable parameters for the FMC is whether FAA or CAA rules are applied. You can trawl through the maintainance pages and find what it is currently set to......