Measure the height of the E in inches. Multiply that dimension by 2.838, then sit that many feet away. (example: if it's 2", sit 5'8" away)
If you can read the 20/100 line, that is your vision, so if you want vision that is better than that, hope you can read the third line.
(from
http://www.eyes-and-vision.com/take-...st-online.html )