Scud if you are cleared to an altitude in a country that has a published transition level, then you set QNH at or just before transition level.
It has been a while since I have operated into a country with "transition level by ATC" stated on the chart, however in this case I believe that if you are cleared an altitude from a flight level you then set the QNH on starting descent.
The information for altimetry in your country would be in its AIP (ours is). I know that for the UK this information is also in the Jeppesen ATC section.
Last edited by Offchocks; 17th June 2002 at 02:58.