I have seen 2 schools of thought on what is considered to be the 1st day of flower.
One is the flip date, which I consider the 1st day, because thats the day you triggered the process.
The other is that Day 1 is when the 1st pistils appear.
Well I don't really agree with that and here is why.
If you let your plants get old enough to be properly sexually active, before flipping, you will already have pistils showing while still in veg, so if you go by when the 1st pistils show, you could be counting veg days as flower days.
I don't flip anything if the preflowers aren't out.
I have had mother plants under 18/6 showing pistils in veg for 3 years, and when you cut clones they already have pistils when you cut them, so 14 days in a cloner and 14 days to root in would mean you are on day 28 of flower, but you are still on 18/6 and just barely rooted in. That makes no sense.
However, the day I flip I also see pistils, as they have been there since veg, so I guess if you veg correctly, flip day is the 1st day of the flower cycle that you see pistils on, so flip day covers both bases.
If you don't see any pistils before flipping, you are flipping too soon. It will cost you on harvest day.
Just because the nodes are staggered doesn't mean she's ripe to breed yet, she needs pistils showing, just as a 13 year old girl can get pregnant, but a 21 year old girl will carry a healthier pregnancy.
That's my story and I'm sticking to it