Even if you rely on a model for mash pH, it won’t give you any insight into pre and post boil pH, or any of your fermentation pH characteristics. You need a meter for that. If you don’t care about those measurements, then proceed as you have been. If you want to dial in, or monitor those variables, as well as, ensure that your predicted mash pH is correct, then you need a meter. Stevie’s example of the acidic best malz lots is a great example of where an assumption based prediction model breaks down vs actual measurement.