How do we get that number?

We calculate it by taking the average of 5 readability equations (Flesch-Kincaid Gravel Level, Gunning Fog Index, Coleman-Liau Index, SMOG index, and the Automated Readability Index).  Each readability equation has its strengths and weaknesses.  By taking the average, we've found that it gives us a much more accurate grade level given the wide variety of text we come across.  We then round that number and display it right on the icon so it's subtle yet useful.