I actually spent an inordinate amount of time calculating the ideal a while back. As a sometime designer, I’m quite intimately familiar with the limits of comfortable reading for the human eye. After a silly quantity of math, I arrived at exactly 115 columns in fixed-width text as the ideally comfortable balance between readability and compactness.

Subtracting two for `diff` output, I hard-wrap all of my source-code and documentation files at column 113 exactly, and have for quite a while. It results in some quite sane output, in my opinion: https://github.com/elliottcable/Paws.c/blob/d2a5f3f/Source/P...

There are, of course, arguments to be made about source code itself; arguments you didn’t generally need to worry about at 78 characters, but that start to come into play when given that extra 35 columns; you need to start worrying about whether a given line of code is sane or readable once it starts to stretch towards 113 characters. There’s some situations where that’s perfectly normal and looks fine; but there’s also several where that indicates a serious problem in your code’s style. It’s just one more thing to think about while you design your source code.