What would be awesome is to include an option for random time, stochastically centered around a certain length, since that is perfect for not being able to game the system. I know that a tool called "Beeminder" had something like this back in the day.
Found the blog post they did back then: http://messymatters.com/tagtime/
I used this for a while and it really was extremely helpful since it activated the "gamey" part of my brain.