I was inspired to create...the Characterization Variation Ratio (C):

C=(#OOC Canons + #OC's) ÷ #Major Canons

Or, in words: You add the number of out-of-character canons to the number of original characters. Then, divide that sum by the number of canons who figure prominently in the fic. The closer the answer (C) is to zero, the closer the characters in the fic are to their canon selves; the closer to 1 C is, the further from canon they are.

As an example, let's use a hypothetical HP badfic which heavily features Harry, Hermione, Ron, Ginny, Dumbledore, Snape, the Weasley twins and a Mary-Sue. In this fic, the Sue figures things out before Hermione, the twins are fighting over who gets to date the Sue, and Dumbledore is ignoring Harry in favor of some prophecy or other involving the Sue.

This leaves us with eight canons, four of whom are OOC, and one original character. For our ratio, this gives us an 8 in the denominator (bottom) and 4 and 1 in the numerator (top). So C=(4+1)÷8 which simplifies to the fraction 5/8. This is a little greater than one half, so this fic's characters, as a whole, are a bit more than halfway against canon.

If all these canons had been OOC, C would have been 9/8 which is greater than 1, signifying a very OOC group of characters. If all the canons had been in character, C would be 1/8, which is relatively small, signifying that the Mary-Sue is maybe the only real problem (barring weird occurences in the story itself).

There are a few things to keep in mind here. This ratio only takes the characters into account. Whether or not the story they appear in is good or bad fic depends on how it's handled; an alternate universe fic could still be goodfic despite having the characters behave differently than in the canon. Additionally, OC's may themselves not be Mary Sues or otherwise fit into the canon universe well. In these cases, you may want to ignore them if they're bit characters, or even include them as canon characters if they are an important part of the story. Finally, if the story includes ALL original characters, you may want to split them between canon-friendly and canon-unfriendly, to avoid dividing by zero (mathematically impossible).

In conclusion, this ratio probably won't work in every case, and it may be a matter of opinion as to whether any given canon is acting OOC enough to be counted (likewise whether an OC is bad enough). Still, it's a relatively quick way to generally determine how far the characters in a fic have fallen from where they should be.

D seems to be hard to compute, but that's more of a sidenote than anything.

There really isn't any unified science to what we do - we write for fun, we pick targets based on their potential for entertainment, and so on. As what we're writing really isn't saving the universes, I don't see any massive need to enforce rules about what is a valid target and what isn't.

That said, multiversal metaphysics are fun. I've always thought of a continuum as a sort of a rope - each story is a strand, and they're generally heading in the same direction. Badfics deviate from that in some dimensional space (as characterizations are at least several dimensions each), and thus weaken the story line. What we do is running around pruning badfics, keeping storylines at least roughly in line.

Also, I'd like to note that there's a difference between deviating from the canon and badfic. There was one fic I read in which the West was North America, history continued about as it has (with the elves hiding when Columbus et all arrived), and the various characters from LoTR reincarnated at some future, post-nuclear-war point. Obviously, said fic couldn't touch the events of canon with a ten-foot pole, but it was goodfic. The biggest complaint I had with it was the amount of angsting that Arwen did.

(And yes, it is 'canon' with a single n. A 'cannon' is a large gun, a 'canon' is the body of work that is agreed upon as official for any story.)

Are you planning to open the department of statistical probabilities? I presume this is the kind of maths the Flowers use to assign badfics to agents. So long as rookies don't have to use it.... I think my heads hurting just trying to cogitate the possibilities. ;)