How should we time a Manly ferry for Sydney Harbour sunset?

Keep a later service as backup, but confirm the last practical return. Do not rely on a forum's timetable for a different season.

Allow time for the boarding queue and choose a service that crosses the open harbour during the light you want. Weather and the direction of travel affect the view.

Check the actual sunset time and the official ferry timetable for your date. Boarding before sunset gives a better chance of seeing both golden light and the city lights later.