Here is the simplest way to set your distance with any HID light hold the back of your hand towards the light slowly raise it until it is comfortably warm and you can hold it there for prolonged time this differs with 2 factors foot print you want from light and type of shades used. If you can get your plants closer to the lights do it will reduce stretch and keep node spacing tighter @Soilgrowth is right depending on shade vented or not and foot print as well as goal 18" is a safe distance.