When to set STD on altimeter
Hi all, I would like some help here. When flying in Europe and taking off from an airport and leveling off ay 3000 ft, the next ATC instruction is climb to FL 210. The TA for the area is 6000ft. For discussion sake, my colleagues are saying that when I am "cleared to" FL 210 that I should make the change but I believe that I should change to STD when passing through 6000. Can someone point me in the right direction. I would like to find the correct procedure, not just a good technique. Thanks in advance.