Yours are my thoughts, exactly.

As far as your comment about the formula values: I agree that they can be calibrated to be more efficient. The I value in particular might need to be tweaked. Perhaps it could be determined based on like the 3rd or 4th standard deviation of the mean average user time spent on a post.

Let's say that the average works out to about 2 minutes and 30 seconds (or 150 seconds) with a standard deviation of about 40 seconds, then post viewing outside of 3 standard deviations (below the mean) would come in at about 150s - (40s * 3) = 150s - 120s = 30s. So we could attempt by starting at an I of 30 seconds. This would make for up to 60 paid intervals within the 30 minute limit, per post voted on, compared to the 180 paid intervals at an I of 10 seconds.