I'm not really sure about that. The amber thing is prolly gonna be more environmental to some degree, heat, humidity, and amount of time on the tree. I look at the time frames as
guidelines. I've harvested early and I've harvested at the 9 week mark with the same strain. I just look at the plants and let them tell me what's up. If you're going to work with the same strain then you can get dialed and really find your sweet spot. The harvest time that works best for your needs.
I'm not looking for a super heavy hitting bud, cause I'm usually pretty active when I smoke, other people might grow the same strain and let those trichs go super amber before harvest. More amber equals more sleepy or so I've read. Then of course you've got the Indy's and Sativa's that also have their own characteristics too.
I don't worry about the veg stage, I just push when I'm ready for them to flower. A big factor for me is the height of my flower tent. I can't let them get too tall or they won't fit the tent...
A lot of people wait until they see between 10 & 20% amber then they cut. Part of the fun of learning!!!
The only perfect time is when you're ready, but you prolly don't wanna go to far past the breeders recommended, final finish date.
The more you grow the more you know!!!!