first of all, the 25% for comment should go to whatever you judge to be the best idea, not number of votes since it can be exploited by the kind of people messaging me on steemit.chat and discord to help with random contests that do the same, it ends up being a challenge of who can spam the most people.

second, the lottery should go to someone random, steemd.com allows you to see the list of ALL voters, where steemit allows only a certain number then says ... and X more. So using that list, you should generate some number then count from top left across until you find their name, simple as that :)