The positioning of the font is made according to the height of the EM square
defined by the font. Some fonts have glyphs that sit on different base-lines
and extend upwards to different heights within the EM square. In order to
position letters so that the top of the letter exactly touches a specific
picel would require individual analysis of the font tables and adjustment of
the output position on a character by character basis. This isn't possible
with the standard methods available to GDI+ and certainly not available to
DrawString.

I have written the code that does the font table analysis to extract such
information from a TTF file but it's a whole lot of work and I'm sorry but I
don't give that away for free.