Light Distance 400 watt MH

I have a 400 watt MH set up, I want to know how close can my plants be to the light. Also the light comes with a crystal tempered glass lens (removable). This unit was not intended for growing but I don't see why it wont work.

Please look at the light and tell me how close you think my plants can be? I have added a cooling fan/vents to this light, so overheating is not a issue. Nowhere in my grow room is it ever over 85 air temp. Can I have to many watts per square foot?

Ruud 400W metal halide multitap, warehouse, shop light
Product Specifications
Product Information
It is the 16" square canopy light direct mount.