I'm pretty sure that most tests have about a 1 year or less shelf life after being opened, I could be wrong though. Does your nitrate test have an expiration date? Most LFS's will test nitrates for you for free. Maybe you could compare the results from the LFS to see if they are similar to the results from your test.
Since your calcium is at 520, what is your alkalinity? How old is the calcium test kit? If your alkalinity is in check then it should be very difficult to get your calcium above 450 or so. I'm going to attempt to explain what is called "The Marble Analogy" as described on on page 183 of Calfo's Book of Coral Propagation.
It is only possible to dissolve so many solids into a give volume of water. Imagine that you have a bowl filled with 100 colored marbles representing the dissolved solids in seawater in a given system. Imagine that the red marbles in the bowl represent calcium and the blue marbles represent alkalinity. The bowl can only hold 100 marbles total regardless of color. The only way to increase the marbles of one color is to displace marbles of another color. Using the example in the book - If 70 marbles represent 400ppm calcium then the remaing 30 marbles represent alkalinity. In order to increase the calcium some of the blue marbles (alkalinity) must be displaced as the bowl can only hold 100 marbles total.
If your tank has a calcium level of 520 (assuming that your test is accurate) then your alkalinity level must be low, according to Calfo's marble analogy. Try testing your alkalinity to see if it is low. What form of calcium are you adding to your tank?
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