50 ppm is the typical value quoted and there are known reasons for that minimum. But I'm speaking of a lower minimum that does not provide all the benefits of the 50 ppm level, but provides at least oxalate precipitation for the wort. We know that beer can be brewed with far less than 50 ppm Ca. However there are advantages to using Ca. The thing that low Ca along with very low levels of other ions provide is a cleaner palate perception for the beer. In many beers, having an ultra clean perception is not important. But in beers like American light lagers and Boh Pils, that clean perception may be welcome. I am suggesting that the 40 ppm minimum is a compromise in cleaning the palate perception and improving some brewing factors.