[kh99]

It sounds like it's very similar to "posts per day" that this site displays, except lower is better. So I guess you'd have to figure out how you want to calculate that. I suppose you could do something like decide that N or more posts per day is "perfect", then calculate the number of posts per day for a user and subtract from N (although then anyone with N or more posts per day would have an "era" of 0, so maybe you need something like 1/PPD instead?).

As for actually implementing it, you'd probably need a plugin to calculate it. If you only want to display it in the postbit then you could use postbit_display_complete and use the values in $post (which should contain the author's data), and set $post['era'] or something like that.