As I mentioned in my overview, I am thinking of ways to somewhat alleviate that issue and give a clearer picture of the amount of more active members and might be implemented in the next month's overview.
I don't think you need to change that. It's OK the way it is. If you were to change that and consider members as "active" if they wrote at least 10 posts in their local boards, you will run into other problems. Although it might work for large subs such as the Russian, where you have thousands of posts each month, it's going to look bad on boards where the total is 100 per month. The stats could then show that the smallest local forums only have 1 or 2 active members.
If you are considering adding something like that, I would suggest not making it a high number. 3 or 5 posts should be enough to call a person active on his local.