However, at the very least you are going to need Calcium in some form.
I do add Calcium, but I wonder how much is really needed. There is also calcium and magnesium in the malt and some is released into the wort. I don't argue that calcium is benefical for beer quality but the benchmark mash that is done to determine extract potential is done with distilled water. The analysis also involves fermenting a sample of the produced wort to determine the wort fermentability which means that fermentation also works w/o calcium from the water.
An experiment on my wish list is brewing the same Pils with RO (~10 ppm Ca), 60 ppm Ca and 150 ppm Ca water. I'm curious to see how the flavor and in particular clarity of the beer is affected by the hardness of the water.