Powerdy mildew on rosemarry
Your best bet is to try to increase air circulation around the plant..either space it further from other houseplants, or try thinning some of the foliage. Spraying it with the baking soda combination will not TREAT the problem (it's used more as a preventative). See this additional information from Iowa State University (it discusses growing rosemary indoors during the winter..about 2/3s of the way into the article): http://www.extension.iastate.edu/newsrel/2003/may03/may0327.html
The one good thing you're doing is having good humidity levels (which rosemary needs), but it is causing the powdery mildew.
Best of luck!