Pilsen water is indeed very soft and the brewers there brewed their beer with that water for many years because that was the water they had. The very low sulfate content of that water allowed them to use large quantities of Saaz hops without the harsh bittering that results if one tries to get 40 IBU with sulfate bearing water. The low levels of calcium, magnesium and sodium gave the beer its renowned soft water character and a very fine beer it is indeed. But who is to say that it is the best beer than can be brewed with Moravian malt and hops from Zatec? If you brew it with Pilsen mineral levels it will be authentic and very good but can it be made better? I think the answer is yes and the reason for this lies, not I believe, in the effects of some extra calcium (which is beneficial to yeast and thus beer in many ways) but in the chloride that goes with it which mellows, sweetens and rounds out a beer. Now it doesn't take much chloride to do the job. I only use 14 grams of CaCl2.?H2O in 55 gal of treated RO water (but blend in a little tap water as well). This gives me chloride at about 40 mg/L. As I have a choice of potassium chloride, sodium chloride, hydrochloric acid, magnesium chloride or calcium chloride I might as well use the salt that gives me potential improvement in other ways and calcium chloride is the obvious choice. I get calcium levels of about 21 mg/L. Appreciably higher than the 7 ppm you refer to but well less than the 30 - 50 ppm that some think are essential.
Were I in your position (and, of course, I was at one time), I'd brew it with really soft water a'la Pilsen and then do it again with supplemented calcium chloride to about the level I mention. See which beer you like better and stick with that approach.
In all of this it is essential that you get the mash pH right so be sure to include some sauermalz in your grist.
