Plants roughly double in height after the switch to twelve hours, and some sativa leaning lines triple, which catches out anyone growing in a cupboard. Expected stretch multipliers by variety type are listed at https://potentthcseeds.com so you can work backwards from your ceiling height. Flipping at knee height in a two metre tent is usually the safe approach. Supercropping mid stretch salvages a bad estimate, though it costs a few days of recovery.