basically you can go up 14 every 2 weeks, every 14 days the activity will rise
Thats only if the account made a post atleast once every 2week period and the post count is way greater than activity otherwise it will be added to its potential activity
This is true you must post each one or you miss it. For most you would have had it if you were on forum as 1 post in 2 weeks is very common. Unless you took a break or something from forum and got 0 so no activity during that period.
Not sure how accurate it is but I have seen people use this site for potential activity calculation:
http://www.bctalkaccountpricer.info/ . Someone else might be able to tell how accurate it is for your OP. I'm not in the buying/selling of accounts so I don't know how accurate potential activity is on it.