In my experience my manufacturer suggests 6"-18" for flowering. I tried it at 6" and I noticed my top bud seemed to start having some orange hairs instead of white, I believe this is from burning. I have since moved it back to 14" and let my bud grow into it, it is now 12" away. I would say at 15" you should be ok... at least my light would be, but yours may differ. If the manufacturer doesn't list a suggested distance maybe try contacting them directly?
Also keep in mind with LEDs the closer you move the light the less light footprint you have. Keep in mind with an LED most are very efficient mine is 98% efficient I believe. A little light goes a long way with LEDs. Think of it as smoking BHO, you smoke a lot less more BHO than you do bud because BHO is 60-80% THC whereas bud is in the 20% range.
For veg if your light is too close your plant will get lazy and just stop stretching vertically, it will either become very bushy or just stop growing altogether. I would think alot of this relies on strain though.