i always wait until at least the plants show maturity by alternating branches in full growth along with preflowers for sexing. usually i give em another 3-4 weeks in veg after the f's have showed using 1 gal pots for the early stages (planting a seed thru preflowers) then the f's go into 5 gal pots or are put into the ground...iow, they're mature and showing sex. alternating branching is my guide for that timing...

and yes...the flowering time given in breeder info is from start of 12/12...but i usually have to add a week or two for actual harvest time.

good luck...