Try checking out this link. I think it can help you out. I personally don't use a glass tank, but many here do with success. Try doing a search for glass tank humidity and that should be able to help you out.

Also, from what I have read on other posts with similar situations, your basking bulb may contribute to the low humidity. Ideally the UTH should provide enough warmth for the warm side of your enclosure, but please, someone correct me if i'm wrong.