start with it farther away,and throughout a couple days,get the light where you want it.i recommend using your hand and feeling for the heat,once its to a point where you feel no more heat,this is where you want your lights. alot that has to do with the distance is how well your venting you heat produced by the bulb. i have my 600watt hps about 18 inches. just feel for the heat though,if its too warm,you know it needs to be higher then that.
