LED Lights, Distance from Plants?

I have a 300w quad band LED light, giving it a trial run to see how it performs, considering I've never used an LED light before, how far should the light be hung over the plants? I know it says it gives off little heat but, I'd like some input from someone who has used an LED before.

I always use the back of my hand for reference, put your hand just were it feels confertable. Then set your light at that height.