Most stuff I’ve read say veg officially begins when the third set of true leaves emerges.
There are so many schools of thought on lighting! LOL!
DLI is the current science. It stands for Daily Light Interval.
The first post in this thread here can explain it far better than I can.
There are free apps for both Apple and Android for measuring the values. They get you close enough.
You can also use height and intensity to help control height of the plant. The lower or more intense the light, the less vertical growth. Most people try to find that happy medium between stout and height.