Hi, and thanks for the question.
This is how I usually handle it. I count Week 00 as the germination phase, from Monday to Sunday, where the seed pops and the first two leaves (cotyledons) appear. After that, Week 01 of the vegetative stage begins.
For flowering, I count it either from the moment you switch your lights to 12/12, or when you first notice pistils forming on the plant, which is often the case with autoflowers.
I hope that helps a bit. In my experience, using a Week 00 for germination makes tracking the timeline much more accurate.
Have Fun and a good Time. 👊