Font specification question (for modifying plftable.h)

Is there documentation for the font descriptors in plftable.h?

The GhostPDL documentation here -

http://svn.ghostscript.com/ghostscript/trunk/ghostpdl/doc/ghostpdl.pdf

- says

"To use a new set of TrueType fonts requires modifying the C code in 
the file pl/plftable.c. The C structure
resident table contains a list of Windows TrueType font names. In the 
released package these names will
correspond with the Windows True Type font names in the URW font set. To 
use a different font set these
names must be replaced with the new font names and the code (at least 
the plftable.c module) should be
recompiled and linked."

I've had no trouble adding monospace fonts to plftable.h (I use a 
licensed Prestige Elite TTF), but I'm baffled by one thing about 
proportional fonts. The code in plftable.h includes lines like this for 
each font (this example is from Albertus Medium):

{0, 1, {313.0, 720000.0 / 313.0}, 0, REGULAR, MEDIUMBOLD, 4362, 27},

My question is: what does "313.0" measure? And, since I'm asking, what 
does 720000 measure? (I understand that the printer internal unit is 
1/7200 inch, but I don't know whether this is another way of indicating 
that same unit.)

Many thanks for any help with this.

Edward Mendelson
Contributing Editor
PC Magazine/PCMag.com