This is how I do it for every brew I do. I'd appreciate any comments. My mash pH is usually right around 5.2-5.3 so I think I'm doing it right. But it seems so simple.
I have the water analysis from the bottled water company that I use.
I go to the spreadsheet and enter the color of the beer I plan to brew in block B8. Say it's about 6 SRM. That will give a range for my target residual alkalinity in blocks C8/D8 (-49 to +10). Now I can skip the 'target water' blocks.
Then I enter my source water which is Spring water. It's VERY important to use the pulldown menu for 'Alkalinity as CaCO3' if your water report is in 'Bicarbonate' (as mine is).
Ca = 58 ppm
Mg = 4 ppm
Bicarbonate = 140 ppm
Sodium = 5
Chloride = 11
Sulphate = 8
pH = 7.7
Then I enter the dilution rate of Distilled water. Since this is a pretty light brew I know I need to reduce the alkalinity of my water by a lot so I cut it by 80%. I'm just trying to reduce alkalinity here.
Next I enter the total amount of water I intend to use in block C23. This brew will use 8 gallons total mash/sparge water. I enter the total amount because I want to treat the entire volume (mash plus sparge).
Next I enter my target residual alkalinity in block B23 (from the first step...just pick a value between the values in blocks C8/D8). My range is -49 to +10 so I'll pick -20 for this one.
The stuff in blue on line 23 displays how much of each water to use and what needs to be done to it and a residual alkalinity range.
Then I start plugging in numbers in the Salt Additions. I know I'll need Calcium to acidify the mash and for this brew I want to keep the Chloride:Sulphate ratio @ about 1 (balanced). I also want to add a tiny bit of Magnesium back because the yeast like it and I've cut my Spring water a lot. So I'll need some combination of CaCl, CaSO4, and MgSO4.
A little trial and error and 3g CaCl, 2g CaSO4, and 1g MgSO4 yields a residual alkalinity of -18 which is close enough. The Cl:SO4 ratio is balanced and I've got my Mg back..I'll take it (this is all on line 39). These additions are per 8 gallons (the volume I entered above).
First thing on brew day is pouring all the water into a big container and treating it.
Looking at it very simplistically, it's essentially a balancing act between Effective Hardness (i.e. Calcium mostly) and Alkalinity. Effective Hardness tends to lower mash pH and Alkalinity raises it. Light brews need low alkalinity but then need some effective hardness to help acidify the mash (so I cut it with distilled and then add some Calcium). Dark brews need alkalinity but not too much hardness (so I add chalk and don't use water that's too hard...mine isn't very hard).