A nitpick: the amount of sit-ups you can do does not correlate well with the six-packness. At 20ish% body fat it's possible to train yourself for a huge amount of sit-ups, but your abs still would not be too visible.
If you have an ability to track your body fat with a decent precision, that would be a better metric.
You're absolutely right. As another friend asked, "is the goal to actually increase ab strength or to receive an unprompted compliment from your girlfriend?" This resolution had what I felt was the least well defined goal/metric and will probably take some refining if I want to "succeed" at it.
To my downvoter, just curious: why? I thought my blog post was relevant, and it seemed better than taking 30 minutes to re-hash my thoughts into a HN comment.
If you have an ability to track your body fat with a decent precision, that would be a better metric.