Robert,
Take a look at
http://links.jstor.org/sici?sici=0035-9254%281975%2924%3A3%3C385%3AAA9TPP%3E2.0.CO%3B2-2&size=LARGE
which gives hints to how the percentage points can be calculated.  It 
also mentions an earlier ACM algorithm.
-Doug
M al