All precipitation is, is the saturation point of calcium when it can no longer dissolve in the water. 500ppm seems to be a magic number for supersaturation, but I don't see the need to achieve such a number; however, the higher you raise your Ca
the more likely you'll also need to raise your Alk
to keep the two balanced. I prefer an Alk
of 4.0-4.2meq/L. The benefits in keeping a higher balance is a much more stable buffered pH, inhibition of various "problematic" algaes, and the rate of calcification is constant.