Encolsed Hood question

when measuring the distance from the tops to my hoods to do i measure from the glass or from the bulb?

Hey man I just measure 18" from the top of the plant to the glass with a 600watt hps works great

Yep - if I measure, it is from the glass. Minor difference from glass to bulb.
But I really don't measure distance, I measure heat from the palm of my hand.
As long as it is not too hot and you are getting good coverage (light spread) from your reflector, keep it as close as you can!