Ive read tons of posts asking the same question. Myself I tend to let the plant decide when day 1 of flower starts. If you havnt let the plant reach maturity and you flip, you will see an extended period of veg like growth until she starts chucking pistils, as I grow from clones, even they show a delayed reaction to start lifting up there skirts and showing pubes. So i tend to start counting flowering days from when they have a few hairs showing. I tend to take breeders flowering times with a piunch of salt aswell due to this. If you were to take the breeders times as gospel, you would be harvesting your girls way too early. ( i tend to grow 8 week strains and regularly let them go 9-10 weeks from switching to 12/12 due to the transition period ). I recently made a cross of Platinum Kush Breath x SFV both by In House Genetics, and that cross has only just started chucking hairs after 12 days since flip, where as the straight PKB was chucking hairs dead on 7 days after flipping the lights, so I count flowering weeks accordingly and tend to harvest at 50/50 milky/amber.
Nothing beats a Loupe to check the trichs, i like to count and check trichs at the same time so i know when to induce stress / PK / Insect Frass / Flush etc so at least i know im in the ball park. Rather take them down late than too early.
To each there own, but i will stress taking them down early is worse than taking them down late.
Peace
Hukt