The measures you can try with the buttons (40, 65 and 90) are to see how the text looks on the edges of these guidelines (which I've written as 50-80 characters).

It's not a scientific number by any means, rather a general "consensus" derived from all the typography literature and articles I've read. The measure is not as simple as it seems, though; Readers seem to prefer a shorter measure while reading speed is actually increased with a longer measure, like shown in this study:

http://psychology.wichita.edu/surl/usabilitynews/72/LineLeng...