I haven't done that recently for Fantasmic. Warning, people start gathering to view fireworks from the castle very early.
My suggestion would be to watch from the Frontierland side of the castle. Traffic exiting the first show/fireworks should be guided out via Adventureland. Traffic flow in will be via Frontierland. You can either walk into Frontierland from the hub or, if it is open, walk through the short cut from the Royal Theater to Rancho del Zocalo.
In the picture below, teal lines are traffic exiting the first Fantasmic show. Purple is if you walk through Frontierland. Red is if you can cut through by the Royal Theater and Rancho del Zocalo.
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In the following picture, red shows the line from RdZ to Pirate's bridge. Blue is if you are coming from SWGE. Yellow is the roped off scooter and standby area where traffic gets congested. This is where I suggest staying to the left.
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