hi tero
the normal or standart procedure for your ? is : when your flying in a FL the controller can see it and will say something like " desent to 3000 feet qnh 1007" when you are req. desent or when you are told. Then you start your desent and then dialing in the qnh.
The reson for is that if you are desenting and waiting to dial in the qnh you need to remember the qnh and the TL and maybe you will forget it and then there is a problem and a lot of papers to be filled out.
best regards
Ran
