Yeah, I can "solve" the problem by aligning to the left and then moving the text box so that it's center.
Anyway, that's fascinating info! I'm not overly code-savvy so I don't know how practical it would be to pull off, but perhaps the issue can be solved without introducing problems by adding a monospacing feature in the future? Like, maybe STM could be set to render a letter every X distance regardless of its actual width, effectively allowing any font to be monospaced.
Thanks for the fast reply! Disregarding this hiccup, I'm having a great time with this plugin.