I'm using a magic ratio on my fixed gear. I used this:
http://www.machinehead-software.co.u...engthcalc.html
You must get an extremely accurate measurement of your chainstay length for this to work, so I would advise calling Bianchi to see if you can get the measurements, or if you do it yourself, try to be as accurate as possible. You might need to file the axle or the drop out a little bit if you mess up, but mine's been working great. I even have just enough room to make up for chain slack.