You can count in different ways proofread pages (3 points)
and validated pages (1 point).
Then you just count (an editathon is short so you don't need fancy things, although there are scripts to do it).
We give the 1st prize to the person with more points,
than we give the 2nd and 3rd prize with a "lottery": every point you have is one ticket, so the more you have points the more chances you have to win.
I encourage you to Keep It Simple for the first time: make sure to check that everyone is behaving and not cheating (ie marking as validated a page that is quickly read), and you should not have problems.