OP, for slightly different reasons, but in the name of allowing flexibility, I am planning to book our FastPasses as close together as the system will allow and not at the extreme end of any of our days. I am going to do some for late morning and some for early evening, but the method in my madness may work for you too: If we get tired/change our minds or want to hop to a different park early, it gives us options. Stringing them out over 6 hours will only tie you to a park for a longer period of time.