It's the
zmax value displayed in the console once you have zoomed/scrolled your overview to the ideal perspective.
In the overview.txt, the third number in the line "ORIGIN" must be the same as the number behind "HEIGHT".
So if you have a line like this:
ORIGIN 453 -226
-514
the HEIGHT would be
-514 as well (if the console showed "514" as
zmax value).
See this thread:
http://forums.bots-united.com/showthread.php?t=2071