I have 66 inches from the soil my plants are in too the highest point the light can be at. I subtract 12 from that because that is the distance my plants will always be from the light. That leaves me with 54 inches. I divide that by 3 because that is how much taller the plant can get during flowering and I see that my plants have to start being flowered at 18 inches tall. is that correct?
They all grow different but thats a good general rule of thumb. I have 2 of the same right now, one is about twice as tall as the other. Exact same nutes, watering and light schedule, one just decided to grow extra tall. (which consequently is the only plant I have had not fit in my box, at 2 weeks into flower I couldnt fit it in there anymore! (I had 44 inchs of growth space total and started flowering when the plants were about a foot tall) The other one still has a good foot or so of free space.