How does that work on a light weight aircraft on a low QNH day when you are cleared to the first flight level (1000ft above TA). Wait for transition and wait for the ATC report that’s filed when you press STD and see you are already above the cleared level !
TL is derived from TA (which is constant for most of the places). TL is the lowest FL you will get from ATC (in terms of units FL with no other factors considered here). this calculation is to assure that the case you described is not an issue respectively separation between aircraft flying in TA and TL is assured. e.g. you have a TA of 10000ft, 1000ft buffer layer and a local QNH of 990. your TL will be FL120. if you switch settings at TA your altimeter will indicate ca. 10600ft at that moment. 1400ft to go to FL120 which is the lowest FL ATC can clear you (again in terms of unit FL no other factors considered here...).
would it be a clever idea to reduce your vertical speed before switching in that example...maybe yes
