It’s foolish to be close minded and judgmental.
Biologically flowering begins when the plant starts to form flowers.
When you want to count from is up to you for your records, but there is nothing accurate about counting arbitrarily from the 12/12 point. It takes 1-3 weeks for plants to transition from veg to flower with many variables effecting it. This is a major reason new growers get confused when their plant isn’t done at “8weeks like the breeder said”. You certainly wouldn’t say a plant was flowering outside until it started forming buds, so I really don’t understand why some indoor growers insist on fighting biology and counting from the arbitrary 12/12 point.
I stopped counting weeks years ago because I learned that the plants weren’t growing by a calendar and we’re only done when they were done.
The first sign the plant is starting to mature is the stigma turning reddish and shrinking into the buds. Once about 90% of them have done that most plants have 2-3 weeks left still. During those last 2-3 weeks the plant may not appear to be growing much outwardly, but is swelling internally. The bracts will individually swell up and get “fatter”. After roughly 2-3 weeks of swelling and ripening you can check the trichomes and determine the final harvest point based on maturity.
If you want to flush I would not suggest doing it before that last 1-2 weeks. There is no point in starving your plant while it’s trying to fatten up. I did multiple side by side tests with flushing vs not flushing as well as having read extensively on the subject, and I don’t waste my time flushing. It never made any difference in the quality of my buds.
The best tasting and smoking buds come from healthy plants that grown to full maturity and then dried nice and slowly.