the only accurate way to track it is when you switch to 12/12, so that is what is used. Anything else is subjective. I can't think of a plant that went to flower before showing some pistils (basically you'd have to not have any preflowers...). Anyway, the stretching, in my experience at least, is going to vary more by strain than light spectrum.
The reasoning for me saying this is that the stretching is not because of the red shift in hps, but from the lack of blue shift in mh (this is a commonly known phenomenon to anyone with much horticulture experience, and is related to the way different spectrum reflects and the ways plants sense if they are getting direct sunlight or not). I run a 400w hps and 400w mh side by side in my flower room, and the plants on the 400w mh side stretch just as much, and sometimes more. In my experience once a plant hits 12/12 for even a day or two, the way it grows becomes more genetically defined, with the exception that better light certainly increased bud production.
