The Standard Line
Research suggests that lines of text should be between 50 and 60 characters.
If you take the frequency of characters in the English language, round to the nearest fraction of 50, and add that many of that character to a line, you get
Whenever I’m designing a page, I take that line, paste it in the main content, and adjust the line-length and font-size accordingly until it just fits.
The line could be better. In particular, I couldn’t find data on the fequency of spaces and punctuation, whose inclusion would improve the effectiveness of the line. The line is better than most methods, though. Different fonts will produce different line lengths given the same font-size, and going by only a single character, like n, m, or x, will miss out on nuance around characters like i or l.