light height

how high should a 600w mh light be above my plants?

This is what I was told a few year back "A good example is that you might be getting 1000 PAR Watts at 4" from your light source, but that would change to 250 PAR Watts at double the distance (8"). This law makes it EXTREMELY important for indoor plant growers to get their light source as close as possible to their plants. The amount of light your plant receives is directly related to it's yield/flower density. "

Also how much heat will a 600 watter put out?

Mogies' point is probably the single most important facet of indoor growing.
As long as you can keep ambient temperature low enough....the closer the light,the better your plants will show their love!

To answer: keep your 600w about 20 inches above the tops of the plants. If you are air-cooling the hood with glass in place you can keep it closer.

To elaborate: the inverse-square law says that a footcandle (lumen) will decrease by half with each foot of distance.

HTH