I'm not sure about this (in fact I'm 90% sure it won't work but it won't hurt to try) but Ashley, if you tried setting the DirectX to put the fonts at "*pt" rather than just the number. I know this is used to specify points size in HTML.
Alternatively, work on the standard of 12pt = 16px high (or 1pt=1px on Mac, because they use 72DPI rather than 96DPI). For every 3pt add 1px and refine it from that. I just work with 14, it's good enough.
Or could you possibly set the application runtime to use px as its font measurement? Or allow users to append pt/px/em etc. to their sizes (this may already work, but I doubt it)