How much of a difference are you getting? Are you testing with a 10ml or 25 ml sample? If the difference is 1ppm or less, I would say consistency in how you perform the test is more important than precise results. If you perform the test the same way every time, especially if you are in the shock process, the reliability of the results will be more important than pinpoint accuracy. By reliability I mean that if you use the same method to perform multiple tests on the same sample of water you will get the same result each time, where as if you swirl the powder 3 seconds on one test and 20 seconds on the next from the same sample of water and get results that are 5ppm or more apart one of those test yielded a false result and is unreliable.
I don't know if I am explaining this well. Sorry if it doesn't make sense.