The Slash Rating calculates the moderated values of a user's recent posts, compared to the maximum possible moderated value, expressed as a percentage.

If all of your recents posts (the ones viewable on your userpage) get moderated up to 5 (the maximum possible per post), then your rating will be 100%. If have a +1 bonus, and none of your posts get moderated, they will all have score of 2. Divided by the maximum possible, this give you a rating of 40%. A user with all posts with a score of 1 will have a rating of 20%.

Why is it useful?
Slashdot posters love thier Karma. But once a user gets to "Excellent" (or "50" when they gave numeric values), it quickly loses any appeal. The only legitimate bonus of having a high Karma on slashdot is +1 bonus to your posts when you hit 25. After that, it's just bragging rights.

But Karma does have it's limitations:

• Obviously, there's the cap. So it's not a true cumulative total.
• Just as obvious, a user's Karma value isn't publicly available.
• Once a user gets a high karma, he gets the "I've got karma to burn" syndrome, and starts to trolling.
• New users start out with very low karma, but can make good posts.
• Most important: Once users hit thier karma cap, the utility and contributory value to the Slasdhot site are indistinguishable. If every post they make is Informative and Insightful and gets moderated up to 5, or just bland enough to *not* get moderated down -- this is not reflected in the Karma value.