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.

My understanding is that it is a weighted average of some sort. I have sometimes experienced what you describe but I suspect it is a miss-attribution -- that I happen to have recently had a post voted up x amount, I check my average karma thinking it "should" go up but it happens to have gone down, most likely not related to that one post. The avg doesn't update continuously. I have no idea what triggers the timing of the updates.

Attempting to be helpful to the best of my ability. Ever since I first saw this I was doing my best to resist the temptation to suggest you must have stood too close to me at some point. But such humor is often a good way to get into trouble.

Best of luck figuring this out and, if you do, hey, give me a holler and let me know what the answer is.

Joakal is true: just don't think about it. A lot of people will downvote your comments based on religion (e.g. if you say something bad about Apple) even if it's true. Just contribute and move on.