It seems that when you look at a feeding chart. The day you go to 12/12 is the start of flower.. but the hard part is when is it done?? But keep in mind that you need to complete week 8 or whatever.. I've known a few good growers that figured out they were a week off cause they waited until the 8th week and not to complete the 8th week. DOH!
The rule is:
Calender / trichs / hairs
You wait the calender, then check the trichs and hairs.. some, like the White I have, will 'look' done by the hair at week 5.. but it's not done.. Calendar first, then start to check the other signs. better to go too long then too short if you ask me.