I have very soft water in North Wales UK, Calcium levels of approx 27ppm. Ignore your water (raw liquor) pH it has no bearing on mash pH or any other pH in the brewing process. I add a small quantity of DWB to the dry grains prior to the mash, this is a formulated blend of minerals.
The calcium ions are what aid the phosphates in the mash to alter the pH. By adding the correct amount of calcium the mash will be buffered enough to lower the pH down to the recommended mash range of 5.1 to 5.3. I measured my mash pH and it was spot on at 5.2. Mash pH is an important part of the brewing process.....well in my view
For Bitters, IPA's, and Pale Ales, the main ions and the approx target range of Theoretical Wort Values Pre Fermentation are: Nitrate (0 - 50 ppm), Calcium (180 - 220), Magnesium ( 0 - 50), Chloride (150 - 250), Sulphate (250 - 450) and Alkalinity as CaCO3 (20 - 60). With my very soft water the Alkalinity as CaCO3 is about 8ppm a lot lower than the target but it wont affect the beer that is made, I could add Na2CO3 to bring this up to target range but the quantities would be very small and not worth it.
You can alter the water profile in BeerSmith to your hearts content, some is detailed in Oginme's response earlier of course. What I did is take the Burton type of water profile that is listed in Beer smith and used this a template for my water and added the ions that are most relevant to the brewing process.
I also add Campden tabs to the total quantity of the raw liquor that I intend to use for the brew before starting. This purely drives off the chlorine/chloramine species in my raw liquor