PLEASE do not use this. Yes, the ideal ("most legible") width for a column of text is ~60 characters, no question. But that's only once you've chosen the most legible font size . You're supposed to adjust your column width to fit an appropriate number of characters, not adjust the font size to your column width! Adjusting the font size is completely backwards, and worst of all, it's breaking my browser zoom , so it's…
As a web designer, I despair of users upping the font size and screwing up the words-per-line. Which is why I like Safari's iOS-like pinch-zoom feature. I do wish all browsers would move in that direction and in the end disallow users to arbitrarily up font sizes. That said, crazygringo's point is valid insofar as people do up their font sizes as a matter of course.
Seriously, where do web "designers" get off these days? The web is not paper.