You can only start using FL when you're at least 3000 ft on the QNH.
If you were to fly FL 35 you would be flying an altitude of 2990 ft.
So I believe it has to be FL 55.
Even if your altimeter is set to 1013 and reads 3500, you are legal, whatever your track is, if the QNH of the day is low enough to keep your altitude below 3000ft.