I think as long as the test is decent ballpark should be fine... if you are testing for a 10.1 and it swings from 10.0-10.2 I don't think that's a big deal... now if the test is like 1dkh off that's a big deal.
I don't know much... but I do know I don't like to stress and if you are trying to maintain an exact precise level you will stress over it a lot.
It sounds like salifert/api/hanna/red sea should all be decent. Pick one and stick with it. Just make sure you test every time the exact same way. For example when you fill up the cuvet, if you fill the water to the bottom of the line do it like that every time. Same when measuring everything else. Always measure to the same level.
That's one thing I don't like about the hanna calcium kit, all those stupid little scoops and powders. but I still use it lol