For our past trips, I've made PS's based on when we planned on eating. Example: if we wanted to eat at the Sci-Fi around 6:00, I'd make a PS for 6:00.
However, for our upcoming trip, I'm going to make the PS's for about 20-30 minutes earlier than when we actually want to eat. Because Priority Seating not an exact reservation system, we've always had about a 20-30 minute wait for our table. Not really a problem, unless we had wanted to catch a show/parade/fireworks afterwards.
This way, if we want to be out of the Sci-Fi by 5:15 to catch the parade, we have more of a chance making it if my PS is for 4:15 instead of 4:45. And if we end up not having a wait, it will be OK, because we'll still be out of there by the time the parade starts.
** OK, this all makes perfect sense in my head, but reading back, it sounds confusing. Sorry if I'm confusing anyone!