as a rule of thumb. what you do is put your hand just above your plant . when you lower your light if you can handle the heat from your light your plant can too. if its too hot rase your light till its conforable.you have to do this as your plant grows every time you adjust the light to the hight of your plant.gl may you be cursed with many buds.I have a question, If HEAT is not an issue, HOW close can you have a 400w hps to your plants.
NICE! this chart holds the key to my answer. +rep for this.Here ya go buddy....