As I've said before, there is a distinct possibility that I may be wrong about FL40. There was just something stuck in my head about that number.
What I am sure of is that the transition level is the lowest usable flight level and as such must be above the transition altitude, hence the transition layer. If you're IFR this is irrelevant as ATC will, in effect, tell you when to reset your altimeter by passing the QNH.