chalmers diagram

I've been looking at John Chalmer's diagram from the cover of his book,
too. It's a great solution to situating the genera in a 2D space. I don't
know if I'm completely getting it though: it seems you call the 3
intervals which add up to 500 cents a, b and c. Anything along the / side
has a=0, anything along the _ side has c=0, and anything along the \ side
has b=0. The distance from the _ side, measured perpendicular to _, is the
size of the interval c. The distance from /, measured perpendicular to /,
gives the size of a. And likewise for \ and b. Consequently the /\ tip
should be all c (i.e. a=0, b=0, c=500), the /_ corner should be all b, and
the _\ corner should be all a. These last two are reversed on the diagram,
so I don't know if I'm understanding it right. Can someone set me
straight?

also, shouldn't the intervals total 498c, not 500c, if we're talking about
Ptolemaic divisions which multiply out to a 4:3? (For Aristoxenian shades
you could argue either way.)

in case you haven't seen it, here's an ascii version of the outline of the
diagram, with apologies to John and the real thing:

C 500
/\
/ \
/ \
a=0 / \ b=0
/ \
/ \
/____________\
A 500 c=0 B 500

cheers --jon