In the case of Fantasmic! (and very likely Jungle Book)-
The FP info will give you a window of when to arrive.
You will be seated in a reserved FP area.
In reality, I would arrive 45-60 minutes before the START-TIME of the performance,
due to the very likely extreme interest in and popularity of this new show,
and the very limited (far too limited) seating available.
One rule of WDW that is virtually always true:
If seeing a particular show is important to you, it is better to arrive far too early,
than to arrive one minute too late.