In cases where we need to convert a logical measurement to a device measurement (maybe including in GdipTransformPoints) I think units_to_pixels/pixels_to_units is appropriate, but it should really take a Graphics object rather than a dpi (since the correct dpi cannot be known without a Graphics object). And we need testing to decide whether to account for the page scale in that calculation.
The helpers are useful for a font object too, that's why they don't use graphics object as input.
Maybe for GdipGetFontHeightGivenDPI it's appropriate, but the other uses in font.c are wrong and should be using the DPI from a Graphics object.