You can easily count it by checking how many post that account has made in 14 days period. You can find a list of dates of 14 days period here --
https://docs.google.com/spreadsheets/d/12saLhlUoqIdairxzuSPu6EYGrt7FN2lOstO1yDjCEbA.
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: User
A 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.