Height of Metal Halide light

My plants are about a month old and about 6-7 inches tall and since they were born they have been under a 125 watt cfl. Today i switched them under a 400 watt metal halide light with a cooling hood. How high should i keep the metal halide light over the plants exactly?

Start high. 12"+ or so for the first day, then lower it until you can hold your hand over the plant and not feel uncomfortable.

you can put the light as close as comfortable for your hand.If you hold your hand over the tops of your plants palms down and it feels hot then raise it.if it feels cool then lower it.since its air cooled you will be able to get it real close without burning them.the closer the better.as big as they are you dont have to wait a day or so go ahead and adjust it.keep an eye on it cause they will grow alot faster thus you will have to raise the light often,