There are a lot of sites out there that can calculate your potential activity (including
mine). Most are based on the same script (but not mine!) that calculate potential activity.
To do it manually, you need to go through the pages of posts and you can use the spreadsheet and information from this thread
https://bitcointalk.org/index.php?topic=582736.0 to find the two week time periods. You earn 14 potential activity for each two week period that has at least 1 post.
do it manually may generate problems for posts too close to the end of activity period, considering that the activity doesn't update exactly every 14 days(it is 14 days plus some minutes)
I think that use a script is better even if you don't care about losing your time