With significant amounts of practice, and proper outdoor lighting, most people can get repeatable results that give the same answer +-5. But the test it's self isn't really that precise. The CYA test is about +-15 when you are doing it correctly. Novices often stop early, when the black dot is kind of mostly obscured, which can give results that are 20 or 30 ppm higher than what they would get if they were doing it correctly. Reading the test indoors seems to result in numbers that are 10 or 20 ppm higher than otherwise.