Depends on what kind of reflector or hood your using ( standard, air cooled, or cooltube)

I will take it you are using a standard hood. A 1000w HPS is around 20 to 24 inches. One way to tell is hold the back of your hand to the light. Start low and continue up untill you feel the heat from the light. As soon as you feel the heat that is the point where you do not want the plants to pass or you will burn the topps of the plant.

JG