what you want is for the light to be as close as possible without causing radiant energy burn. That is, the closer, the better. But too close, and you could have the leaves become too hot, regardless of air temperature in the room, from radiant heat energy from the light (below the visible spectrum). So go with what you have now, and check it often, if you notice that its getting the leaves too hot, move it up. Best suggestion, 2-6 inches more than that, and you'll be wasting energy by having it too far away.

Peace and Pot