The method seems to be rather complicated. In my profile pages it says I been a member for 4 days with 16 posts so shouldn't it say 4 posts/day? Instead it says 3.25 just before posting this. I'm just curious.
Since you became a BF member on Sep 12, and u had posted your 16th post at 8:42pm of Sep. 17, your member period equates to 4.86 days approximately. If you divide 16 by 4.86 you'll get 3.29 posts/day. However, i don't what time on sep 12 that you had registered in BF.