What I did in the beginning.... Was cut a small chain 18 inches, then connect that to the hood between the light and the plants...
it will help prevent you from getting the lights to close. =) Once they get a bit older, they can handle the "hand test" in other words, put your hand under the light raise it to the light when you feel heat, the plants feel heat, then you set the tops to the point where you felt the heat and not closer than that or you will burn the plant.
After that, you'll get the hang of it, as it becomes second nature.
All of this is of course, if you're running a HID.