Plants will stretch 2-3x's their size when flowering, if you put 12" plants into flower they could end up being around 24-36+" tall, maybe more with poor conditions. Take whatever room you have between your pot and light, subtract for plant canopy to light clearance, this will vary depending on what light/hood you have 1000W=24", 600w=20", 400w = 16", cfl/fluro/led could be within a couple inches, air cooled hoods can get even closer, my 600 cool tube i can keep my hand on after its been running for hours, then divide by 3..this should be about when you want to start flowering. So If you have 60" between the top of your pot and the bottom of your light, minus 12" clearance, equals 48" total flowering room, 48" divided by 3 would give you a theoretical 16" tall plant you could put into flower. Unless you have grown, or KNOW the characteristics of what your growing, I'd flower at 12-14" just to give a little more head room.