CMH Effective Distance?

For my next grow, I plan on going with Northern Lights #5 x Haze. From what I understand, the plant can stretch quite a bit with a large main cola. My grow light is a 315W CMH. I keep it approximately 18 inches away from the canopy. Does anyone happen to know what distance my light would be effective at? With a large main cola I don't want to find myself in a situation where I waste perfectly good bud since my light fails to reach far enough. Thanks!

Those things pound out around 30,000 lumens. That's a good bit. There's a chart here:



I would say that you're just about right with that 18 inches, but you could lower it down to around 11 inches and get a bit more out of it (provided you have the coverage, of course) without any real risk to the plants.