CYA is the most subjective test of them all--differences in lighting, distance of tube from eyes, length of time the solution sat on the counter before you poured it in the vial, etc, can all affect the test. The best thing you can do is try to be consistent..i.e. use the same lighting, same distance, same time of day, to be as consistent as you can. It's entirely possible that your CYA did lower, due to splashout or dilution.......

Janet