Transition Level is either:
Nominated and published to the plates
or
Selected by ATC on the day
It is not calculated
If the transition level is listed as "By ATC", in the descent, should you change at the controller's first clearance to an altitude or should you change at the transition altitude. This has been a re-occurring question among crew members and to date, I have not read anywhere with a definitive answers. Maybe some of the ATC types can help