That's why you need to find a LBS with a good reputation for fit, as others have said. Then, testing bikes that are at least roughly set up for you, you can get a better sense of what feels right. It may be that those lower-end Bianchis with the 3 cm gap between sizes may not be right for you. A friend had a bike that was too big for her, and the shop that sold it to her had compensated by putting on a tiny stem--it looked almost like the bars were attached directly to the top of the fork itself!