How high do i need my light?

i'm getting this Sun System® HPS 150 Grow Light Fixture from Growco Indoor Garden Supply 150w HPS to light a 12"x12"x30" grow space.

I was wondering how high i should place the light? If the light should be placed below the 30" limit when the plants first start off and raised as they grow, where should the initial light placement be?

18-24 inches. u might have to move the lamp every day by an inch or 2 keep in mind its not always once a week or something like that