I suddenly realized that I don't remember what numbers end most musicals, perhaps because the movie adaptations change everything. "One Day More" in "Les Mis" certainly tells everyone that something major will take place in Act II, but it nonetheless has problems: It wraps up one story, and then Act II starts another, to me less interesting story. Yet, I can't think of another place to break.
I've seen versions of "The Phantom of the Opera" where the chandelier falls towards the end, right before the final lair scene (The Las Vegas version, which is presented in one act, and the movie). Maybe because I saw the movie first, followed by the Las Vegas production, followed by the original stage show, I think the falling chandelier is SO major that it belongs at the end. I think ending Act I after "Masquerade," and crashing the chandelier before the final lair would be preferable.
You alone can make my song take flight.