I use almost the same procedure but I store the cuvettes with rodi water and no air gap, this has greatly increased the life of the cuvett before replacement, they need to be replaced when any marks or scratches show up. Most people don't keep their equipment clean which is one main testing error issue, then the tester gets blamed for being inaccurate.
I also check one test sample with two alkalinity checkers and most times both checkers read the same value or a .1 difference from each other.
With all their testers it takes multiple test to get a rhythm and procedure down to preform it the same way every time, then a trend is noticed. This is when they become a great tool.
Some people use the test kit a few times a year and probably don't preform the test the same or let to much time go by since opening the reagents even though the exp. Date hasn't been reached. I like to use the kits on a regular basis and replace the reagents often this keeps everything fresh for more accuracy.
Ultimately use the test kit that works for you, that's why there is so many options not that an alkalinity titration test is difficult and one is better than another but maybe someone can use one test better in their situation over another. So luckily we have options im glad there's not only one brand!