Flowering terminology

I think this is down to personal preference. I would say flower day x is when lights have changed to 12/12 on a photo. For an auto it is when you see the first preflowers, for me. This is important as this is a massive change in the plants growth and environment and when you will perhaps be changing humidity and nutrients, important to log everything you do to help determine where and why they is an issue when one pops up. 👍🌱

autoflowers are a guesstimate too. indoors you count from flip, so that it's just consistent.. nothing technical about it, really. When the hormonal balance in the plant flips is when it matters.

I think outdoor might be as you describe. Indoor growers some times simplify by saying flowering is when you switch the lights to 12/12. In my case, starting from clones, they always have pistils because they're taken from a mature mom, so I count since the switch too.