Personally I never check the PH, PPM or anything other than quantity of the run off. Why check something with so many variables that could result in a false reading. Input controls output, so if you  use good media, and PH the water/feed and flush you have controlled the input. Monitor the results by looking at the leaves, or if you want to go over the top check the Brix level and the sap PH. Why stress over what you cant really control unless you really like stress.