ok, well if the final pot is gonna sit 10 inches high, and the light needs 8 inches (7+1 so the plants arent touching the light), that leaves 30 inches for the plant. I would start flowering when the plant is 15 inches tall. You can adjust this if the pot is gonna be smaller or bigger.

Also, look into LST (low stress training - bending the plant over). I dont really know much about it but alot of people on here do it, and you can grow a plant alot bigger with the same height restrictions. I'm not sure if your box is wide enough for that, maybe someone else will post here.