One thing to do is test your water supply. There are generally only 2 sources of water for most pools (aside from trucks used to fill): city water (or well water) and rainwater. Rainwater is going to be about 0 for calcium and your tap water will vary by region (ours is 110). But if you are filling with tap water when the levels get a little low, that represents the lowest amount that you should see for the pool calcium level.