I used a threshhold of 35 at first, then I upped it to 50. These days you probably want 75 or 100 if you really want to sift through the less important items.
Or you can just subscribe to daemonology's RSS: http://www.daemonology.net/hn-daily/index.rss. This summarizes the top ten every day. These are most of the important articles; of course you will miss some important things.
On the other side of the scale, you should try http://news.ycombinator.com/newest every once in a while as well. The fact is, the HN point system ends up filtering a lot of things by whatever's popular or in sync with the groupthink at the moment. Newest can help you avoid this "filter."