Created 2011 May 11. Updated 2011 May 11.
Regarding legibility, a rule I heard at FOWD London 2011 during Eliot Jay Stocks' presentation was roughly: line length related to line height in that the longer the line length the greater the line height. Reason being that the longer the line length the harder it is to track back to the correct next line. By providing a larger space between lines this is made easier.