Water pH is not mash pH
Two waters can have the same pH and very different alkalinity. Water and grain chemistry jointly determine mash pH. A water report and beer colour alone cannot guarantee an exact acid dose or mash pH.
Measure a cooled sample
Calibrate your meter according to its manufacturer, typically using suitable pH 4 and 7 buffers. After thorough mash-in and an initial reaction period, take a sample and cool it to room temperature. A common guide range is pH 5.2–5.6 at room temperature, with the preferred range also depending on the recipe. Automatic temperature compensation does not replace cooling: actual sample pH changes with temperature.
Record corrections
Record timing, sample temperature, pH and acid additions. Make small corrections, mix thoroughly and measure again. Account for existing acidulated malt and acid. The water-treatment estimate is not an additional blanket mash dose.
Treat sparge water separately
Sparge water encounters different buffering conditions from the fresh mash. Treat its alkalinity separately using the actual sparge volume. A water-pH target without alkalinity information is unreliable. No-sparge recipes need no sparge-water treatment.