Whence 50 percent?

The thinking seems to be in some circles that counting student test scores for anything less than 50 percent in teacher evaluation won’t get a state Race to the Top money. Does anybody know where the 50 percent figure originated? Is it just because it is powerful to say that at least half a teacher’s value be tied to student performance? It seems like a lot to me when the tests and processes still, by everyone’s admission, need to be refined—so I am wondering how this figure has become the rhetorical baseline.