Your goal is out of balance, you should be shooting for more like 425-450 Ca to match your alk target of 9-11 dkh.
Trying to push the ca higher is going to be problematic and will provide no benifit. It's likely that you are adding too much ca and causing at least some precipitation which then messes with both levels. To use Calfo's not perfect but useful analogy, you have a bowl that holds 100 red and blue marbles. You are trying to fit 65 red and 50 blue in the bowl at the same time, they don't fit, 15 have to fall out.
If you use the reef chemistry calculator, you can enter one value and it will tell you the other level in balance.
http://reef.diesyst.com/chemcalc/chemcalc.html
For example, at 11dkh your balanced Ca is 439 ppm. At 9dkh, your balanced Ca is 425.
Also check your mg level if you haven't already. Mg affects the Ca alk balance and if it's out of range it will make it almost impossible to achieve and maintain a normal balance between Ca and alk (but it doesn't sound like this is your problem at this time).
Finally, if you were able to maintain 500 ca and alk anywhere in the range you listed you might want to check on how you are measuring salinity. When the salinity is elevated beyond natural levels, the water can hold more Ca and alk, but those levels will still have to be in balance. In some cases with swing arm hydrometers and similar gadgets you can get a creep effect over time as tiny amounts of salt accumulate in or on the gadget impacting it's readings. Again, I don't think this is your problem at the moment, but it is something to be aware of particularly if you are tempted to push the normal range of these parameters (i.e, if you are able to push the Ca that high and keep it there, you probably have a salinity problem).
