So you're getting a real light huh?
OK, so what you should do is look at the amount of headroom ya got. Lets say you got 3 feet.
Figure the light for 6 inches and 8 inches away for a total of 14 inches. So you got a little under 2 feet to grow in. To get the most out of your plant you should go MH for a week, let her get used to it. Then top the plant and let the side branches catch up probably about two weeks then flip to 12/12 with the plant at about 18" tall. She will finish at about two feet and you will have two top colas and larger buds on the side.
Better?