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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)