You're talking about probability of upvote, but that needs to be multiplied by the number of people who read the comment, and that depends heavily on where it is on the comment page itself. When I look at my karma on my recent comments they have very little to do with how much time I spent on them or how unique my perspective is... I even wonder if that aspect is gameable sometimes, whether you could guess the probability that with your upvote a comment that has been recently posted 5 minutes ago could be boosted above the top comment posted 5 hours ago, and if the 15 minute ago one was interesting enough maybe it would be self-sustaining up there, and you could potentially produce the second comment in the page by replying to it. But that's a lot of math when probably if you're trying to optimize karma/comment there are better heuristics, “always comment on any post that is on the front page with < 20 points and < 20 comments” or so, just increasing volume of things that are probably in the right place to be upvoted.