With no advertising and very little publicity, it now shows up close to the top of Google's search results for queries like "percent off calculator."
I slapped some (kind of obnoxious) advertising on it ... but for HN users, here's an ad-free version:
http://pressbin.com/tools/percentage_calculator/?HN=
Just make sure to enter something for the HN variable -- your HN username, some random characters, a URL-encoded haiku, etc.