I too had a simlar LBS experience--first bike too big, LBS recommendation based solely on standover height. Standover height is a necessary but not sufficient condition for a good fit. I got a 50 Bianchi Eros Donna several months ago that fits perfectly--I had help from a local physical therapist who is an expert cyclist and cycling coach on the fit. Top tube length really matters. See this article, too:

http://www.myra-simon.com/bike/womens-fit.html