Today I brewed for the first time with distilled water. I've always used my tap water - very good - but have often had trouble hitting my target mash pH. We had another thread recently where we were discussing "errors" with the calculator so I decided to test it. Remembering, mash pH is a function of the buffering power of the mash and the buffering power of the water. The water calculator uses several factors to determine the mash pH, one of which is the water report. If you use municipal water, the water will vary over the course of the year. That was my thought when I couldn't hit mash pH so I decided to brew with as pure water as I could get. The results: Nailed it. I did a step mash with a predicted mash pH of 5.3. First step, pH was 5.11. Second step 5.24 due to dilution. The finished wort? I did two tests, cooled. One came out at 5.30, the other at 5.29. I came out one point short on gravity but what a beautiful wort! Granted, this is an n=1 situation but at this point I'm sold: If the beer comes out as nicely as I think it will, there's a RO rig in my future.