There is an issue with the scale calculation above. If you are talking about a rectangular map of a spherical world, then the scale of 1cm = 1354 km is only true at the equator. The closer to the poles you get, the greater the distortion. In effect your ratio scale approaches 1to1 till 1cm = 1cm at the poles. That is because the top and bottom neat lines of the map are actually two different points, the north and South Pole respectively. To make a long story short, follow Feanaaro's advice. Scale is elusive on a global map that isn't an actual globe, unless you have a program that allows you to recalculate your map at different projections.