I keep mine under flouresants intell their about 5" tall, mainly because its cheaper, the lights are on anyway for the moms i have.

When i go to HID's ,Ill start about 2 feet away ,and move them down to about 1 foot in a weeks time, I use vented lights, you'll have to check your temps,under the lights.

I add more perlite to my MG soil also, I think most problems with MG are from overwatering.