Light bleaching & distance?

I've got 3 Himalayan Gold girls flowering and one is showing light bleaching on a couple of it's fan leaves. I was wondering why all of a sudden it's happening when the light is actually farther away from the effected leaves than just a couple of weeks ago. Also, can the amount of nutes contribute to light bleaching? I'm using Fox Farm and upped the amount of nutes to full strength from 3/4. I am also using a air cooled, enclosed hood for my 600 W HPS so there is hardly any heat coming off the light (you can put your hand on top of the hood and it's cool). Also, I've heard conflicting posts about how far to put the light from the tops of the plants. I've heard anywhere from 18 inches to as close as you can without feeling alot of heat (which in my case would be just inches). Which is it? :stoned: