I wouldn't overly concern yourself with the formula as it is confusing, but basically it's saying you can only get 14 activity points every two weeks and to get those you need to make the 14 posts. However, you only need to make one post in each activity period to get the potential merit? and you can collect the rest later by making the required posts. Everything is explained here: https://bitcointalksearch.org/topic/faq-everything-you-need-to-know-about-forum-activity-account-ranks-and-merit-2766177
Potential merit? You mean potential activity.
Yes, i think he meant activity and not merit. If you make one post in 14 days, you will get the potential activity. Later when you complete the posts (30 Posts for Jr Member), you will rank up (if you meet the merit requirements).
There is no potential merit sort of thing. The more merits you get, the quicker it will be for you to rank up provided your posts/activity requirement are fulfilled.