Look at it this way, many of Shimano's cranks only come in 165-175 size. A difference of only 1 cm but yet that accommodates people that range in size from say 5 ft tall to 6.5 ft tall. Ball park it, find what's comfortable, and then worry about something more meaningful. And btw, there are very few individuals that can tell the difference in 2.5mm, the width of a nickle.