Need help with controlling plant height

I'm growing Northern lights and Bubbelicous this time around, both are supposed to be Indica dominant strains with shorter plants. I topped some of them at 1 foot but they seem to make up the distance by increasing internodal distance. I keep a 400w MH about 16-18" away and have noticed a little leaf curl on top of the tallest ones when I forget to move it up. I was running 21/3 but dropped back to 18/6 about 3 weeks ago, I only fertilize twice a week (at the recommended dosage) with MaxiGro by GH. Keeping the light at 16" allows me a maximum height of 3 ft 10". I really just want to get them to sexual maturity then 12/12, knowing that they will double in height during flowering. Any suggestions on cutting down on internodal distance? Less nutes? more light?

well try FIM ing them next time also 400 watt mh is a lot of light so its gonna be hard to keep them short in such a small space so my sugestion is try lst (low stress training) theses plants are very versitile they will almost grow any way u want them

im having a similar problem. Im considering letting them grow for whole month more and then chop them in half and double my plants

Chopping your plants will not double them. Esp in flower or late veg. Youll end up with half a plant and a dead half plant. Maybe even 2 dead half plants. Bend your plants gradually over a period of a day or two. This will lower your profile, slow the growth as hormones are being redistributed, and more bud sites will be exposed to light. You can bend all the way thru flowering, if you do it slowly in small increments.