With that water you don't need to add anything for a dry stout. Provided you have about 10% roast barley the pH will go to about 5.5 which is fine. You could add some additional chloride and/or some additional sulfate which would also have the effect of increasing calcium as, presumably, you'd be using the calcium salts but that is not really necessary. It is, however, something you should experiment with in order to find out just how much of those ions makes the beer you like best.