Zoo Med does give a chart listing distances. I believe that is rather an effective distance chart. A T5 HO should be kept further away than a T8 since they produce more UVB. I would say 6-8 inches for a T8 and probably around 12-14 inches for a T5 HO. But really without using a solar meter, it's all guess work more or less. Now for MVBs, a 100 watt should be 12 inches away and a 160 watt should be 18 inches away. Regardless though, UVB tubes really wont put out enough UVB to adversely hurt your dragon unless you get it really close and that is because of the chance of UVC which is dangerous but has a very short range. Now an MVB can do damage since they put out about 4 times the UVB of a tube thus having a longer range for the UVC.
Also can I ask why you got her in a 15 gallon tank inside of a 4x2x3 cage? I mean other than the chance of finding food being harder(if you feed crickets and just as easy if you feed dubias or phoenix worms), I don't see any advantage to the smaller cage. I started Harley off by partitioning off half of her 4ft cage for like 5 days just so I could make sure she was eating when I used crickets and she was 6 weeks old but after I switched to roaches, I opened her cage up fully and she loved it. She would sprint full blast from one end to the other and then jump up on her bricks to bask and do it again. Trying to get a temp/uvb gradient in a 15 gallon tank is close to impossible because remember, your dragon needs somewhere that they can get out of the UVB if they choose to, kind of like using shade on a sunny day outside.