derive a percentage for the node as the number of votes received to the number of monks having existed in total

You'd probably have to check for votes received to the number of monks who logged in that day/week/etc. Theres lots, and lots, and lots, of monks who have never even cast a vote.

Perhaps it might make more sense to just keep track of the total number of votes cast in a day, and compare that to the node rep? (Isn't that basically how the xp system works anyways?)