If it is not sexually mature and you have seen no evidence, I would go from when I first see flowers. If it is a clone or has preflowers, you can probably go from the flip. It does not in any way shape or form take into account the vegging time. This is the time after the flip. The flowering time is also just an estimate and that time is for as pretty well dialed in space. It could well take a week or 10 days longer than the breeder estimates.
I hope I don't sound mean here, but if you did not know this, it would be a really good idea to do some more reading. This is pretty basic growing knowledge. It is important to know and understand the life cycle of cannabis and what it needs during these times. Otherwise, you are going to find this a tough go.