Karma per post is not well correlated with the actual quality of your contributions. You can get hundreds of karma for posting superficial observations early on a post that become popular. Someone that has observed HN for long enough could probably get 1000 karma in a day from just trying to optimize for karma instead of quality.
If you really do want to reach some karma threshold I would suggest trying to get it from submissions instead. You can easily get hundreds of karma per submission if you submit the newest release of some popular software product or the latest Zed Shaw rant with very little invested. This also has the bonus that you are not actually lowering the quality of your commenting for more karma.
That, and doing some basic research to post additional useful information (facts, numbers, stats, references, background info, etc) on a relatively new thread.
I'm mentioning this because I believe such posts are genuinely useful and surely deserve the points they get.
BTW I don't think I ever got near a hundred points for a comment. But I suppose it can happen in the above circumstance in a popular thread.
If you really do want to reach some karma threshold I would suggest trying to get it from submissions instead. You can easily get hundreds of karma per submission if you submit the newest release of some popular software product or the latest Zed Shaw rant with very little invested. This also has the bonus that you are not actually lowering the quality of your commenting for more karma.