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.
Hidde de Vries (@hdv@front-end.social) is a web enthusiast from Rotterdam, The Netherlands. He currently works on accessibility standards for the Dutch government (views his own) and is in the W3C's AB. Previously, he worked for Mozilla, W3C/WAI, national and local governments, Sanoma Learning and others as a freelancer. Hidde is a public speaker (all 79 talks). In his free time, he works on a coffee table book covering the video conferencing apps of our decade.