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

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