JeroenC:
If I understand your question correctly, you are asking for confirmation that you always have to use FL above the Trans Alt.
This is not quite correct, as it is approved practice (different SOPs notwithstanding) to set QNH and fly by reference to that when cleared to an altitude below the Transition Altitude even if you are physically still above it (i.e. in FL area). You must be not expecting level reports to be requested until past the T. Alt, however.
There has been some discussion on other threads about the advisability of setting QNH very early in the descent (or 1013 in the climb) because of the risk of getting an intermediate level off or level check request and the danger of levelling at the wrong level or reporting the wrong one. Some airline SOPs have other criteria for when to change for this reason.