Links
The ch unit for line lengths
Richard Rutter explains that maybe the ch unit isn't the best choice if you're trying to follow Robert Bringhurt's guidelines:
The important part of Bringhurst’s guideline is not the ’66-characters’ but the ‘satisfactory length’. This is about readability, and readability is affected by the length of a line more so than the number of characters in it.
(From: Use of ch unit considered inappropriate (in certain circumstances) | Clagnut by Richard Rutter)