Hello,
Right now I have a 20 gal cage. On top I have like chicken cope as the lid. And I have the 100 wat basking bulb from exo terra hanging from the ceiling of my room so I can adjust it to the tempatures. I also have a uvb (reptisun 10.0 ) I do not know the fixture. But its placed above where the basking bulb is on the cage. Is this fine? I don't know when but at some point I'm going to look for a bigger cage for him. But my main question is will he be fine if the uvb is where he is. He seems pretty happy sometimes he has stress marks but when I take him out they seem to go away and sometimes if the lamp is to hot he will move away and darken up a little bit until I come and notice then adjust the lamp a big higher.
You'll need to upgrade a 20g soon because it's much too small for an adult dragon so you can address getting an upgraded UVB light then. For now, if you have a T8 and a reflective fixture you can just keep it on top of the tank and it should be OK. You'll want it to sit about 6'' above the basking area for best results. The screen top will intercept a little bit of UVB but if you mounted it inside the tank, it would take up too much overhead space in a tank that size.