Setting time before cut is not about pH (in the sense of targetting pH), it is about targetting a final moisture target in the cheese, which is determined by the gel strength. The gel strength is determined by milk pH, rennet amount, and set time. A very helpful was to combine all three is to use the idea of flocculation. I've written about this before, and Debi posted a helpful chart with common ranges for cheese styles. If you search for flocculation, you can find our past discussions.
Using clean break is a very subjective way of doing it.
You don't measure pH before curd cut because that marker is rather pointless. Again, you're after a specific gel strength. It doesn't matter what that pH level is if you standardize milk and add rennet at 6.5-6.6. Then you cook the curds to release whey and try to get the temperature+ stir schedule to get you to a final moisture target in the curds so that the moisture target coincides with the pH target. A range of pH targets are valid for cheddar for the drain pH... from 6.0-6.2 or so. The more important aspect is again moisture.
For cheddar, you target a moisture content based on what kind it is. For a long-aged cheddar like a 2-3 year old, you need a lower moisture target or about 35%. For common commercial 90-180 day aged, it's more like 37%. This makes a big difference. I can use the exact same culture and pH points but different moisture and salt targets and get completely different cheddars through aging.
