I suppose that should a FMS fail, nowadays you declare an emergency...?
!
Honestly, I rarely use the FMS for my descent planning. As soon as you get a shortcut during approach (e.g. during the transition to one of the runways in FRA), you might end up way too high if you had followed the FMS, which calculates your descent based on what you entered - usually the whole transition.
Have an idea of what to expect during the approach, pay attention where ATC turns the other aircraft onto final, and - most important and already mentioned by others - pay attention to what your training captain says (with the experiance you have on type, the training captain should be your first and preferred source of information. Come back for more later
). He should know. The rest will come with experiance.
Have fun,
DBate