Disregard the height unless you have issues with how tall your plants get. You should allow your plants to grow for at least 7-8 weeks or until they show alternating nodes before you begin the 12/12 cycle, or you could let em grow for 6, that's what I did. But I'd imagine you'd go ahead and give em the few weeks. I am on tight schedule and had to get done before a certain point. Still didn't make it but I'm close to it .