Glad that got clarified, because the original post had me scratching my head.
With the F! Dining package you use your voucher to enter the amphitheater any time up to 30 minutes before showtime, and seat yourself in the center, reserved section. You bypass guests entering standby. If the theater is approaching capacity, any unoccupied seats may be opened to general seating 20 minutes or less before showtime, so don't just show up minutes before the show is due to start.
If you have a FP, you will have an entry window that allows you to enter and then find a seat in the FP or the standby section. As long as you enter within the window, you will have a seat, even though it may not be in an area you might want.
Robo gives some very good advice about F! in this thread:
http://www.disboards.com/threads/question-about-fantasmic.3409519/#post-53649371