I do it by the size.
I currently bloom at 6-8" so they max at about 2.5 ft.
If you bloom at a foot they will easily be 3-4 ft tall at the end.
It is all relative though to your lights and how you fim, prune, supercrop, etc.
It is also all relative to how much you are trying to yield as well and how often.
If you are doing personal vs. business its totally different. I am like 50% personal here so its not important for me to "maximum yield" and also inherently "maximize risks"... weigh your needs/wants. Big plants need big lights and create BIG SMELLS. LIKE WHOA. I had 3- 4 footers that easily stunk up a 2000sq ft house.
Play with it.