I use a rather elaborate spreadsheet which I developed more for academic purposes (for a course I did for my homebrew club) than for practical use. You are welcome to it but brace yourself. It's at
www.wetnewf.org. There is a User's Manual that goes with it that is currently at 64 pages and growing.
The only time I use it for actual brewing is if, as in the example of the course I mentioned above, I want to brew 2 ales - 1 with "authentic" Burton water and the other with unmodified well water so the class can see that the water does really make a difference. For my regualar brewing I know how my well water performs. For Pils I use RO water with about 20% well water. For most other beers I just use the well water. I watch pH in the mashtun and tweak with HCl but more recently I've started using sauermalz - much more Rheinheitsgebotish. My philosopy is: get the mash pH right then add the other salts to taste.
