While the 66-character/per line is a common ideal setting, you cannot lock it to that, as the characters used have varying widths, especially when considering various different languages. A different approach is to measure the width of a lowercase alphabet for a given language,
abcdefghijklmnopqrstuvwxyz in English, and set the textwidth to about 2.5 times this.
The information is paraphrased for The Elements of Typographic Style by Robert Bringhurst.
In the code below, edit the alphabet to language you are using.