Its fine if the UVB is 5 inches away but, I highly recommend that there is NOTHING between the bulb and the dragon. Anything that is between them will greatly reduce the amount of UVB that your dragon desperately needs. Since your using a repti-glo it is even more important that there is no barrier. The Repti-glo does not put out nearly enough UVB and there has been some health concerns with it as well, IE: Eye-irritation.
When you get extra cash invest in a Reptisun 10.0 tube available at petmountain.com, Your dragon will love you for it.