I think it's something like: (Total Points / Posts) / Days since registered.
Don't worry about karma anyway. There are some people who will downvote you even if you're completely right. Don't delete it if it's really contributing to the thread. Some people disagree with reality.
No. pg has posted about this several times. I can't find one of those comments with a casual search, but it's pretty close to: start with the most recent 50 comments you've made, throw out the highest-scoring one, average the remaining ones. The focus is to average over your recent activity.
Just an empirical observation, your average karma seems to recalculate on a periodic as opposed to a continual basis. I've noticed at times that, after posting something that gets a lot of upvotes, checking my profile (even when I don't hit my browser cache) doesn't show any increase to my average, yet waiting a day or so does.
This is purely anecdotal and unscientific, but it's a consistent observation.
Don't worry about karma anyway. There are some people who will downvote you even if you're completely right. Don't delete it if it's really contributing to the thread. Some people disagree with reality.