Yes, in John Doerr’s book, at the end there is a precious section “More Litmus Tests” with a bullet item about KR:

Make sure your key results are measurable: It must be possible to objectively assign a grade at the end of the quarter. “Improve sign-ups” isn’t a good result result. Better: “Improve daily sign-ups by 25 percent by May 1.” John Doerr, p 264 Measure What Matters