What Rejection Numbers are Good?

Miss Snark wrote a post on her blog that inlcuded the following quote:

You got 3 [requests to see the manuscript] from 40 queries. That's not bad....That's actually pretty good.

I keep hearing people complain about only getting five or eight requests for partials (with subsequent rejections). Of course, they never said how many query letters they've sent out. In any case, I began to wonder just what numbers were considered "pretty good." Now I know. Three of forty works out to 1 out of every 13 agents requesting a full manuscript.