Is this formula for approximating the character count per line in a div correct?
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I came across some [website](http://www.webtypography.net/Rhythm_and_Proportion/Horizontal_Motion/2.1.2/) that says

<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>

The CSS for its _column 1_ is `DIV#col1 { width:400px }`

At the moment, I'm not so concerned with some case scenario where the _reader increases the text size_.

I don't understand where he gets his ratios from, but this formula seems to account for both ratios he gives:

    approximate charcount per line = [2 * (div width)] / (font size)

The units in the formula are pixels...

Further down he gives two more examples, and the formula still seems to hold...

Is this formula correct? Why is he giving those ratios?

I would think that someone asked this before, but maybe I have the wrong search terms :)