Unfortunately there is no perfect answer. 12 inches is a rule of thumb for the most common type of UV light (fluorescent tube). If you are using this type the answer really is "the closer the better but no further than 12 inches). There are more and less powerful UV sources available. There are specialist spot lamps sold only to zoos and large scale keepers that can throw UVB up to 5 feet or more. If you use regular flood mercury vapor bulbs then you usually want something right around 12 inches. I have heard reports that some of the regular spot bulbs can put out quite a bit more UVB and you probably wouldn’t want to go less than 12 inches with those.

The problem with giving specific distances is that UV light follows the inverse square law and attenuates with distance. Also the molecules in our air attenuate the uv radiation even further (I was unable to find a reference to exactly how much air attenuates UV). Then you have screen cage tops which can greatly reduce the amount of UV received..

If you are very serious and want to make the investment then a UVB meter is the only way to know for sure how much UVB you are getting. For reference I get around 50 uW/cm2 from my 6 month old 40 watt reptisun 5.0 fluorescent at 5 inches. At 8 I get only about 10 uW/cm2. From my new MVB i get an incredible 1500 uW/cm2 at 1 inch from the bulb face but at 12 inches it has dropped to around 300 uW/cm2.

Never mind the comment about air absorbing UV radiation. It does but I found my reference here and it is only 24% for every 1000M in distance. This is insignificant at the distance we are talking about.