title: Is this formula for approximating the character count per line in a div correct?
body: <p>I came across some <a href="http://www.webtypography.net/Rhythm_and_Proportion/Horizontal_Motion/2.1.2/" rel="noreferrer">website</a> that says</p>
  
  <p><i>In the preceding example, column 1 has a fixed width: it has been set to 400 px wide. With text rendering at 12 px this would result in a measure of approximately 66 characters per line. If your reader increases the text size to 16 px then the measure reduces to 50 characters per line.</i></p>
  
  <p>The CSS for its <em>column 1</em> is <code>DIV#col1 { width:400px }</code></p>
  
  <p>At the moment, I'm not so concerned with some case scenario where the <em>reader increases the text size</em>.</p>
  
  <p>I don't understand where he gets his ratios from, but this formula seems to account for both ratios he gives:</p>
  
  <pre><code>approximate charcount per line = [2 * (div width)] / (font size)
  </code></pre>
  
  <p>The units in the formula are pixels...</p>
  
  <p>Further down he gives two more examples, and the formula still seems to hold...</p>
  
  <p>Is this formula correct? Why is he giving those ratios?</p>
  
  <p>I would think that someone asked this before, but maybe I have the wrong search terms :)</p>