Grebs,
in my case it was too late when I discovered light was to close. Trics were already turning amber (ie like 70%), so the only solution was to harvest at like 6 wks., into an 8 wk. bloom period. so I only got 10 ozs, but I was educated !! So this grow I eliminated the hanger that comes with it, and chained it to top of tent, 3 inches from top of (6.5) tent, so it was like 4.5 foot from the 7 week old clones , when I started, and now it's about 3 ft. from canopy, stretch is done and I'm feeling good about it at this stage !! But I think the one 315 bulb should be able to be at 18 inches, if you can position a fan, blowing across the light spectrum. Like I say, I've been burned once, and not I'm very pernoid about getting any closer to canopy, but I'll say one thing, It puts out a ton of par, from way up there, and everything is growing and filling out nicely. Gotta remember, I'm only 26 days into bloom.
If I was you, i'd hang that light about 3 foot from the baby's, or I'd more likely put them under my t5s or cfls, as that's all they need at that stage. We're like the minority, using these lights, and like I say, the jury's out, and if this don't work out, I'll probably go back to hps . And this will be just another expensive mistake !! Ha !! let's hope not !! This is the fan I have positioned to blow across the spectrum.
I'll be watching your baby's grow !!