Hi Oliver,
the T/C is calculated when you press the EXEC button - and then it remains static, even if the airplane climbs much better (or worse) than expected. It is rare to reach the top of climb at exactly the precalculated point in the real airplane.
Whenever you do a recalculation (like flying direct to a waypoint, entering a new climb speed, etc.) that requires you to hit the EXEC button, the T/C will be recalculated.
In the real plane the T/C symbol will also disappear as you fly past it, this does not happen (yet) on our model.
Cheers Jan