I usually try to keep them about six inches away. Three years ago, I was experimenting with a red, blue and red/blue panel and had it a little over and inch away. One plant curved, I mean really curved, to grow toward a blue bulb!

IME, these panels simply are not worth their cost unless you practically stole it! Four 23-watt CFL bulbs will easily cover a 20x30" area and only cost you about 78¢ more in electricity over 100 hours of use.

Mike

*Sigh*. Well I figured as much, alas I bought it on ebay for $50 after reading an amazing glowing review. Immediately after my purchase I read another review (and others) which said this product was a dud. I can be impulsive :(
Ok, you can stop laughing now! :S

Turn around, re-sell it, and use that $50 for something better ;)