Base unit of writerly fame = the Trout. One trout is roughly equal to any non-self published publication up to pro-paying publications, which are worth two Trouts. (Sorry, the problem with solely self-published authors is that they are by definition unlikely to be known by a non-fan, so maybe we could call them Salmon?)
A MegaTrout is someone with 1-3 novels/short story collections published by medium to large presses (medium as defined by Subterranean, Night Shade, etc).
A GigaTrout has more than three novels published in such fashion.
After Giga Trout, we enter the Scalzis, since Brian knows Scalzi personally, this was the easiest way to explain to him. (He did not in fact recognize the name Rowling, which threw of four scale entirely.)
Keeping in mind that two Scalzis make a half Gaiman, and at least two Gaimans are required to enter the next level, the Legend scale, which included canonical authors like Ursula LeGuin, Jane Yolen, and Ray Bradbury, this should allow us to pretty well pinpoint most authors.
Now, in addtion, we have the Doctorow Effect and the Sagan Factor.
The Doctorow Effect is a unit of internet fame. It can make the perceived rating of an author much greater than their actual literary rating, due to internet renown. Hence: "Author X would be a GigaTrout, but the Doctorow Effect bumps her up to a half Scalzi."
The Sagan Factor is very important in Science Fiction. It kicks in when the actual scientific renown, academic cache, and/or accomplishments of the author dovetails with their literary output to make them very hardcore indeed. Geoff Landis, Rudy Rucker, and David Brin are good examples of these.
Any additions? Leave them in the comments. We go forth!