API is OK for calcium. I always got reasonable values using it. My main issue was losing count of the drops! I wouldn't recommend the Seachem calcium test as the color change is hard to see (although I like that it uses a syringe for the titration so that you do not have to count the drops).
John Coppolino, a well known hobbyist in the US, only tests alkalinity in his amazing SPS tanks.
http://www.3reef.com/forums/sps-cora...l#.UktN7L9TvFI
IMO for reef tanks that consume substantial amounts of calcium, testing for alkalinity and calcium is very important.