For HID lamps. (Metal Halide-MH, High Pressure Sodium-hps)
1) Place your hand (palm down) at top of canopy.
2) Lower light until you cannot stand the heat for a full minute...once that distance is determined...
3) Raise lamp 2" and keep that distance consistent throughout grow.
Note: Always have a fan dedicated to the airspace between the lamp and canopy, and you will be safe. An oscillating fan is necessary to circulate air.