With todays screens being wider than 1024px, many text containers tend to have a width greater than 600 pixels. I had earlier thought that text should have a width of 600 pixels less for faster horizontal eye scanning and did not notice margin sizes also affected readability.

The link on http://hubel.sfasu.edu/research/textmargin.html states that its not only text width but also margin sizes that affect readability.

Since most websites have a margin width between 10px to 24 px, a column width of 576px (8 inches on a Windows PC although I find 552px to be more widely used) for text columns seems to be the best option for websites. For margins of 27px onwards, a text column width of 288 px (4 inches) seems to be optimum.