Seedking...

Its about the heat your plant will feel from sitting in the same spot for hours on end. Your hand will only tell you if your hand is getting burned.

It depends on the air temp, circulation / ventilation environment and the use or not use of a reflector.

For example, I can get my HPS in my hand and it wont burn as long as the fan is right on it. But after a minute or so I can begin to feel the heat rise.

Plants are the same.

Just start out high and work your way down. Keep the temp guage right between the lights and the canopy spacing.

In one of my threads, I get the MH within 2 inches of the canopy....but as the air temp changes because of the garage, I need to adjust it accordingly as well.

Place safe and work down....starting at 18" without a reflector and at least 24" if you are using a reflector.

Good luck man