It might be better to make that proportional -- e.g. that 20 upvotes needs 30 flags. I think there are enough "junk" upvotes (catchy title that people upvote without reading, somebody's favorite topic, but didn't read the article, new users that aren't quite used to the HN vibe, etc.) and now a sufficient number of users that getting 10 votes probably isn't all that hard.