Hey Sandy!
To answer your question I have 40gal to 60ish gal. I use the 125watt Exo-terra in all of them(
live in California if I lived in a cooler state I'd probably need the larger wattage on the larger viv's or atleast more added wattage of extra light to acquire temps). I don't have any problems with keeping temps within range. Dawn is correct I don't use screens as I'm always home or someone is to keep an eye on all the beardies. But like GoFast said if you use a screen it needs to be metal and a large mesh perferably. The ones that normally come on most viv's are not the kind of screen to use if you don't want to block UVB. Sandy your right the best way to adjust the basking temps would be to adjust the height of your basking spot. Myself I have mine clamped to sticks that are mounted to my stands...they also make a lamp stand for the clamp on hood lamps.
Anyway for right now don't use the PowerSun 100watt there is testing being done on that bulb. They changed something in it and didn't change the packaging so the distance needs to be different or something like that you can read more about it in Tracie's thread.
viewtopic.php?f=6&t=105629
I've spoke to Frances Baines Tracie's friend and she assured me that the Exo-terra Solar Glo was a good bulb that she is even using one. She hasn't added the Solar Glo to the uvguide yet but will be adding it soon. She was kind enough to share this with me so I'm going to share it here.
However I realize the packaging on the MVB's say to change every 12 months myself I don't have a UVB tester so I always change mine at about 8 to 9 months. Better safe than sorry. Not only is this a good bulb but I find it to be well priced here.
http://www.reptilesupply.com/index.php?cPath=26_33