Really there is no need to count from any point, as the plant will not be done based on a calendar. It will be done once it has finished its life cycle based on the environment you provide to the plant.
I personally do not consider a plant to be flowering until it begins to show flower formation, the nodes stack tighter, and the first bursts of flower growth begin. I feel this way based on my experience growing 1k+ plants the last 10 years and from the reading I've done about plant biology. It takes time for plants to build enough of the correct hormones to actually make a switch to the flower cycle. That transition can take anywhere from a few days to a few weeks depending on the strain, the maturity of the plant AND your environment. That few days or weeks makes counting from 12/12 totally arbitrary since it can vary depending on conditions like I mentioned.
However if you simply use basic observation you can look at a plant and tell within a couple days whether it has began to form flowers. If you aren't looking at your plant well enough every 2-3 days to tell, you are already doing something wrong. Growers outdoors have been doing this forever. There isn't a magic light timer outside for growers to flip, so they look at their plant and when it starts forming flowers it has obviously began its flower period.
