Pure water is neutral at pH 7.0 only at 25 °C: its ionisation grows with temperature, so neutral is about 7.5 at 0 °C, 6.8 at body temperature and 6.1 at 100 °C, and water at 100 °C is not acidic.
Neutral means equal hydrogen and hydroxide ions, and Kw = [H+][OH-] is 10 to the minus 14 at 25 °C but about 10 to the minus 12.25 at 100 °C, so each ion is at 10 to the minus 6.125 and the pH is 6.1. The same shift moves every pH scale: at 100 °C a pH of 6.5 is alkaline, not acidic.
The shift is for pure water. It is not the same for the dissociation constants of acids, which have their own temperature dependence, and a pH meter measures a temperature-corrected electrode potential, so quote the temperature with any pH in a hot process.
Estimate with the rule, then check it against the calculator that models it properly.
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