Embedding TrueType fonts in PDF

I hacked some more on the PDF backend, and now it embeds TrueType
fonts. The positioning is still a bit off, but what looks like a
bigger problem is how to get the /Widths array required by PDF. I've
tried both the width and horiAdvance properties of font.load_char(ch),
and in either case Acrobat Reader tells me the array is invalid.
Opening the file in Mac OS X's Preview, adding an annotation and then
saving the file changes the array to something acroread does not
complain about (and it also subsets the font). It doesn't look like
it's a simple scaling problem, since the ratio of correct width to
either of my guesses is not constant.

Any ideas?