Another variation on the theme....
ICAO doc4444 says that a level can be assigned to an aircraft when the conflicting aircraft has left that level.
example CPA002 "Left F250", we say "GFA 751 climb FL 250", couple of caveats there somewhere about similar performance aircraft etc. Not what we used to do in OZ I can tell you
then (and possibly now) we had to have a minimum of 1000 feet.