Flowering Time question

Hello, I was wondering when you consider flowering to have begun. When the lights switch 12/12 or when first sign of sex are shown?

flowering starts when u flip to 12/12

Use both. Track the day you switch, or the day it shows clear & positive gender signs. The difference between these could be seen as your range. For indoors I'd lean with the photoperiod, because as soon as you give them scheduled 12/12, importantly the 12 hrs darkness makes the sex hormones start to be produced & flow. There's no gradual change like outdoors.

I figure you ask this because of the time given for a strain to mature. The best way is to use a microscope and examine the plants & buds individually. This way you can play a role in more precisely tuning the high to your liking.