Calcium levels generally only go down as you replace water, depending on the CH of the fill water.

Since you have a gunite pool, you need more calcium. You can add it directly, or you can chlorinate with cal-hypo--only use the 68% chlorine type. It will chlorinate your water nicely, and add the calcium you need, albeit slower than directly adding it. Still, by sometime in the season you should be well into the 200-400ppm range and can stop.