This procedure is a standard by now in all Europe,at least.
If the controller will tell you to descent at ,for ex. 4000' ,with TL at 60 ,when you are at FL 150,you should set QNH,no matter what TL is.
Then if he asks you what level are you passing,just tell him your altitude -8500' for ex.He just wants to verify his transpoder indication on the screen.
The only problem is if the ATC want's you to mantain FL 90 for ex.Then you should reset Standard and hold at FL indicated.
Brgds Alex