I don't see a need to have a high calcium content in brewing water, except as needed to balance elevated sulfate, chloride, bicarbonate content. I do not know what the minimum calcium should be, but I know that one of the major American brewers that I have consulted with uses a nearly Pilsen-like calcium content of around 13 ppm. I'm sure they would have beerstone problems if they didn't have a very active cleaning protocol. With regard to a minimum that I would use...I suggest that 40 to 50 ppm calcium would be safe and provide adequate calcium for the precipitation reactions.
With regard to the upper calcium limit, it will depend upon the other ions that you desire in your water. For instance, Burton water has huge sulfate, chloride, and bicarbonate concentrations that would be difficult to balance if you didn't allow high calcium concentration. Increasing the magnesium or sodium concentrations to balance the high anion concentrations would quickly produce undesirable flavor in the finished beer. Many of the water profiles from world brewing centers have calcium concentrations approaching 100 ppm and Burton and Dortmund are over 200 ppm. Big numbers, but still suitable for brewing.
Another thing that I've recognized for water profiles, the cation content has to increase as the bicarbonate content is increased for darker beer styles (increased RA for darker beer). Again, you should try to keep the magnesium or sodium content relatively low and increase the calcium content as needed to balance the increased bicarbonate concentration that is needed to buffer the effect of the dark grain acidity. So, the bottom line is that water for darker styles should have higher calcium content than needed for pale styles.