If the no. of posts made in all 14 days period is less than 14 * 'no. of 14 days period account has made at least one post', then 14 * 'no. of 14 days period account has made at least one post' - current activity = potential activity. Potential activity can be claimed by just posting regardless of the time.
Example: UserA has made 10 posts in 10 14 days period, then it's current activity is 10 * 10 = 100 and potential activity is 14 * 10 - 100 = 140 - 100 = 40. You can claim this 40 potential activity by making 40 more posts.
P.S. Please lock this thread if you have got what you wanted. Lower left 'lock topic' button.
It's easy to do that way but is very time consuming and you could make an error making it less acurate. I think a bot would be great and very helpful if it could be made. I know I'd use it because I can still earn some activity points by the looks of things and would like to know how many without going through my entire post history.