This is a big debate, because imo plants dont magically start flowering the next day when u switch schedule, when u switch they need to do the transition from veg hormones to flower hormones, they need to build up flowering hormones to start flowering, usually like 7 days then the stretch, if i see preflowers before switching the light schedule then i count since switch, but if i force flower switching schedule then i count when i see preflowers, that way im always really close at what breeders say. But i guess everybody have their own personal growing style!! Remember this is not rule of thumb or anything im talking out of my growing preference opinion and experience. And i respect when anyone wants to start counting flowering stage.