I adjust the Czeck Pils to 50 ppm Ca using CaCl2.
AJ and I just had a discussion about Pilsen water and the American Lager water profile I have included in Bru'n Water. Either of those profiles have very low calcium that defies the current thinking on the minimum calcium in water. As we know, those beers tend to have a degree of delicateness that is probably due in part to the low mineralization in the water.
It appears that there are a couple of reasons why you could get away with a much lower Ca content than 40 or 50 ppm. First, I've stated in the past that it appears that a minimum of 40 ppm Ca is appropriate for reducing beerstone formation. We also know that 50 ppm Ca is helpful for improving yeast health and flocculation performance. In the case of fighting beerstone, the mega brewers have probably instituted strenuous clean-in-place protocols that reduce the formation of beerstone in the first place. We homebrewers would have to weigh if we want to incurr the wrath of beerstone by reducing Ca much lower than 40 ppm. In the case of yeast health and flocculation, we know that big breweries have no problem in pitching huge quantities of yeast. Therefore, yeast health and growth are not a big concern for them. We also know that big breweries have elaborate settling, fining, and filtering procedures, so they don't really have to worry about flocculation in their brewhouse. Craft and home brewers might need to pay more heed to this factor.
So it appears that you could work around lower Ca concentrations in your brewing if you are willing to deal with the hazards.