Fortunately a bike that's close to the right size can be adjusted to fit. Most people can comfortably fit within a two size range. I can ride most 56's and some 58's. Heck, there's probably some 54's I an ride. Unfortunately fitting is not an exacting science. Science can put a fitter in a good starting point, but individual preferences and individual medical issues will likely have to be accounted for. Fitting shouldn't be looked at as a one time thing. A few adjustments may have to be made after you spend a little time on the bike.