Research suggests that lines of text should be between 50 and 60 characters. The average length of words is about 5.
If you take the frequency of characters in the English language, round to the nearest fraction of 50, add that many of that character to a line, then add a space on average every fifth letter, you get
ohien r uyidainbt og tetsrahe lhetdla emif srkstw coe nepao
This line is exactly 59 characters long, and should roughly match the length of english text. It’s fine if one or two of the last words overflow onto the next line.
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.
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.