It is possible but you must do it manually. I would suggest that you write down (or make a spreadsheet) of all of the "activity" periods since you have joined (when they started/ended). Then go through your post history to check as to how many activity periods you posted in. You then multiple that number times 14 and you have your answer
That's exactly what i meant above but some said we need more than that.
Because, you also have to factor in the default number of activity that you missed. (A.K.A, the amount that you missed no matter what.) Not to mention, they registered in 2011 which would make it only harder. Activity came in at around the end of 2012, beginning of 2013, or something like that. You'd have to see what the activity would be in the old system, in which posts calculated your rank. (If I remember correctly.)
I believe that when the activity system was implemented, none of the "old" activity was transferred to the "new" system, as a result some people's ranks changed either up or down based on what activity level they have with the new system of getting up to 14 activity points every two weeks they post