Optimum Canopy Height from 1000w

I've a couple of 4x4's with a 1000w cooled hood and experienced some bleaching recently. It seems that the plants closer to the 'hot zone' of the lights are exhibiting more stress/curl and bleaching than elsewhere which is more healthy dark green.

They are approx 22" from the glass on the hood, temp ranges from 79-83. There is a fan blowing air towards the top of the canopy to keep it cool.

I would like to know what is the optimum canopy height in relation to a 1000w hood, or a approximation with temperatures, environmental factors, etc.

Im running CO2 if that helps any.

Thanks.

The plants should be a bare minimum of 17" away from the bulb at all times. 21" is a good distance to set the bulbs away from the plants, while still providing a few inches of growing room. This is all only with sufficient cooling of the lamp. We use outside air channelled through the lamp with a centrifugal fan and then straight out the grow room. Nine months out of the year, this keeps the lamp nice and cold while the grow area's air remains room temp.