Environment and overall plant health can shorten, or lengthen, the duration of bloom from the onset of 12/12, to finish. For this reason, I feel it is imperitive to go by a set point, so that the information presented is easily recognizable as relative. For instance, grower A goes by day 1, growers B-D go by initial showing (2-14 days in), growers E-Z simply skip a week. With the vast discrepencies in this information, it is impossible to ascertain environmental factors and plant health. With a set point, the correct one (I might add), it is easy to see who's grow is exemplary, and who's is in need of a little "assistance", as well as where those deficiencies may be.
Another factor that is not a "given" is the duration of lights out. Some do 12 hours, some 24, 36, etc. So, it is imperitive to go by the initial day 12 hours of darkness.
The only set point that we all share is that at some point we expose them to 12 hours of darkness and at that point they begin to flower.