The proper place for this would be the parchment bug tracker.
Does using ch's instead of em's work?
Nope, according to http://www.w3schools.com/cssref/css_units.asp
, which lists "em" but not "ch" as a valid unit of measurement in CSS. Apparently "em" in CSS isn't defined as the width of an m anyway; it's defined as the point size of the current font, regardless of how wide or narrow the font is. Which is a decent enough proxy for whatever it is we're actually trying to measure.
I couldn't find a .css file in your Github repo that looked at all similar to this one, so I figured it wasn't proper to file an issue about it on Github... besides which I don't see an "issues" page there, so I'm actually not sure where the Parchment bug tracker lives.