Unless you leave your seats at Fantasmic when the boat with all of the characters on it makes its way into the Fantasmic Theater, it will be practically impossible to get to a spot before Jingle Bell Jingle BAM begins.
Fantasmic is approx. 27 minutes long when I have timed it.
I had the same situation my recent trip. I left my seat when the boat came out...watched the boat sail by from the top of the theater...When Sorcerer Mickey came out, I then started walking at the top of the theater toward the exit...The show was over, and I got a spot for Star Wars: A Galactic Spectacular about 30 seconds after it started. I was booking it though.
If you get stuck in the traffic leaving Fantasmic after the show from your seat, you will most likely miss half of Jingle Bell Jingle BAM.