Thanks for the responses.
For the font in question, I think I managed not to mix controlling it with ems and %s – ems are used elsewhere for the template’s standard font.
Even when I set font with just a fixed px size (e.g. 16px), the font seemed to be different sizes on mac and pc machines, even when using firefox for a browser on both machines (on my pc machine, firefox had imported my IE settings, on my mac machine, firefox had likely imported safari settings – but all had default text size of 16). I’m a little confused if that is normal or not. Is it?
I eventually gave up trying to “fix” the inconsistency, and chose a font where the difference in size was minimal (but not zero) – Times New Roman. I’ll probably just stick with that unless someone has a suggestion. Thanks again.