Once the grains and mash salts are well mixed, take a pH reading by placing the probe tip of the ph meter directly in the mash. The ph meter we recommend has automatic temperature compensation (ATC) up to 140F so we do not worry about cooling down a sample as any errors introduced will be minimal as we usually measure the mash pH when the temperature is just slightly above 140F. While the probe electrode will last longer if the sample is first cooled to room temperature, ours (so far) has last 10 years when brewing once or twice a month, so we've never bothered cooling down a sample as it's much quicker. Feel free to cool a sample to room temperature if you prefer.
For optimal starch to sugar conversion we want the pH to be around 5.2 to 5.4, but only when measured in the mash temperature range of somewhere around 145-165F. pH changes with temperature, so if the sample has been cooled to room temperature, a pH of 5.4 to 5.6 is the target range instead of 5.2 to 5.4. Don't confuse the inclusion of automatic temperature compensation (ATC) in the ph meter to mean that the target range will always be 5.2 to 5.4 regardless of temperature, as that would be incorrect. ATC only compensates for the change in the electrical response of the pH meter probe with temperature. It does not compensate for the chemical/energy change in the water that naturally makes a hotter mash more acidic (lower pH). The pH target range will therefore always depend on the temperature of the sample: 5.2 to 5.4 when measured at mash temperature, or 5.4 to 5.6 when measured at room temperature.