class PDF::Reader::FontDescriptor

def glyph_to_pdf_scale_factor

any behavior, but uses units/em to define how wide the 'M' is (the widest letter)
PDF states that a glyph is 1000 units wide, true type doesn't enforce 
def glyph_to_pdf_scale_factor
  if @is_ttf
    @glyph_to_pdf_sf ||= (1.0 / ttf_program_stream.header.units_per_em) * 1000.0
  else
    @glyph_to_pdf_sf ||= 1.0
  end
  @glyph_to_pdf_sf
end