To me the 40 drops suggests that the operator only "guessed" how many drops are enough, to the nearest multiple of 10, and left that as good enough; and rhe 38

To me the 40 drops suggests that the operator only "guessed" how many drops are enough, to the nearest multiple of 10, and left that as good enough; and rhe 38 suggestion was basically saying, "find out exactly how much is enough instead of being ok with good enough"