Set CSS width to given number of characters

I have a block of text in a <pre> element rendered with font-family: monospace. I want to set the width of the <pre> element to be 86 characters wide, the length of the longest line in the block.

width: 86em doesn’t work here because it takes the current element’s font-size, multiplies by 86, and sets that as the width. This would work if, for a font-size: 13px, each character was exactly 13px wide but in my experience that won’t be the case.

What works, instead, is width: 86ch which is the width of the “0” (zero) character. But because we’re using a monospace font, “0” will be the same width as all the other characters.