Comment 4 for bug 795130

The reason "M" is better is because overall font metrics include EVERY glyph, even ones that extend an unusual amount outside the "norm" for the font. I suppose you could use "font size" to come up with something relative to the baseline, but that will require some tweaking to get results that look good across a variety of fonts.

Also, "M" is most like what "everyone's existing schematics" is already using, and is a decent representative of "average glyph size" even if the "M" (or other iso-latin-1 glyphs) doesn't occur anywhere in the string.