Measuring pH at home is not very meaningful. This is because the data we would read in the pH-meter will be the sum of the 2 acidities: Lactic (produced by lactic acid bacteria) and Acetic (created mostly by yeast).
The critical factor is the proportion between the 2 acidities. We could have a sourdough with a correct global acidity (e.g., ph 4) but with an excessive lactic or acetic acidity.