Here is how it is calculated according to the Unofficial list of rules here: https://bitcointalksearch.org/topic/unofficial-list-of-official-bitcointalkorg-rules-guidelines-faq-703657
A:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Activity is updated every hour.
For example, if in a 14 day period you post 14 posts, you gain +14 activity.
If you post 2 posts in a 14 day period, you gain +2 activity.
If you don't post at all in a 14 day period, you don't gain activity.
However, if you post 15 or more posts in a 14 day period, you still gain only +14 activity.
Some extra reading on it: https://bitcointalksearch.org/topic/til-activity-does-not-work-on-precisely-2-week-intervals-interval-timings-582736
Oh, I see. Thanks for the help. Closed thread.