Hello and Greetings,
I also have very soft water in North Wales, Calcium levels of approx 27ppm. Not sure what u mean about the mean pH.....is this your Raw Liquor? If so, ignore water 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 Malt 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.
What I have done is sent a sample of my water for analysis and (see the excellent BrewUK website and Water Analysis) it costs about ?29 and I had results back for all types of beers in addition to what I have mentioned above: Stouts/porters/Milds and Lagers/Pilsners. I think its well worth it.
I am not sure how effective the water profile is in BeerSmith. 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. But I still don't know if it matters in the grand scheme of things in BeerSmith...
Hope I haven't confused you...