We had a combination of the methods above:
- when climbing to FL we set 1013 at trans alt.
- when descending we changed to QNH upon getting the first descent clearance below the TL.
Recently there was a change in the SOP and we are now required to change at TL, EXCEPT when we've been cleared for approach earlier - then may change to QNH above TL. Simple, isn't it?
I was told (and even checked it, but don't remember now), the change in the SOP followed changes in the ICAO DOC 4444. I don't have it at home now, but I think that's wher you can find "the truth"
Greetings
Smola