Back when I was studying typography in college the general wisdom was that readability was best when the line length was somewhere between 36-42 characters. It varies a bit with font design (character width relative to x-height, length of assenders and decenders, ...) and how the type is set (leading, added letterspacing, ...). I have mild dislexia and find that, in general, line lengths on the narrow side of this range work best for me.
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