I've only used Red Sea and Salifert. Red Sea always comes out to a 400 level (the test measures calcium at even levels of 50). The Salifert generally has some variation in the level and measures in levels of 10.
I do use Red Sea salt which claims to have a calcium level of 400, so maybe the test results are accurate, but when I see no variation it makes me doubtful of the accuracy. Note that the Salifert test has never been more than 20 off from the 400 level (Range always tests between 380 and 420).