Hi Richard! Many guests will start forming the stand-by line for Fantasmic! about an hour before show time so when you finish your dinner, I would suggest you head over to that line. I do think you will have enough time provided there isn't an unexpected delay for your dinner reservation. Personally, I would arrive for the reservation a few minutes early, just in case they have availability to be seated a few minutes early.
Besides the Fantasmic! show, you can enjoy
Wishes Nighttime Spectacular at Magic Kingdom. Anywhere on Main Street near the castle is a great place to view the show. I also like to stand on the bridge to Tomorrowland so I have a great view of Tinkerbell flying through the sky!
Illuminations: Reflections of Earth is a must see at Epcot. As long as you have a clear view of the World Showcase Lagoon, you will have a great spot for this show! My favorite spot is in the Showcase Plaza area but if you are staying at a resort in the Boardwalk area, you may want to view the show from Canada or United Kingdom which will give you easy access to the exit once the show is over.
I hope your vacation is truly magical! Diane