As I have never used GTK+/gtkmm before, I spent a few days reading documentation to familiarize myself with the library, after getting stuck on transitioning parts of the UI to
Inkscape::Util::Unit. With a better understanding of how the UI worked, I was able to make good progress and have completely eliminated
SPMetric from the code base.
This leaves one significant section of unit code to port,
SVGLength, which I hope to finish by the end of the week, as well as a couple other minor sections of code. I’m a few weeks behind schedule but hope to finish the refactor before the end of the month.