https://www.brewersfriend.com/mash-chemistry-and-brewing-water-calculator/
So, reporting back. For my Dubbel, I set the above water calculator to 5.3 mash pH. It calculated a sparge pH of 3.7.
I just took all the actual measurements.
The actual sparge (and strike) liquor pH before adding to the grain was measured at 4.4. So kinda close?
The actual sparged wort pH was 5.45 when I took a sample.
The actual mash pH was 5.38. Could be a refractometer calibration error, but reasonably close.
Not sure why it measured lower (perhaps sample stratification?), but the combined wort pre-boil pH was 5.3.
I do think I will set the pH closer to 5.2 next time, but overall I am pleased with these numbers. I have never taken all these pH readings, but I will from now on. Getting the pre-boil pH between 5.2 and 5.4 is the sweet spot, I understand, for the best hop utilization and smoother bitterness.
As for the calculator predicting the low sparge pH, I think it's just a bug/error? Or maybe it should be changed to say sparge liquor pH.
One thing of concern is what happens when it predicts a higher sparge pH, such as 5.5? Will that push the measured sparge pH above 6? For my process, I think the wise move is to see if I can get the the calculator below 4 for the sparge, assuming the predicted mash pH is in line.