Great question. Hope somebody knows..lol. I tried to check the ph on some dunder from the last multi gen cm fiasco. Only to figger out all my test kits use eye chart comparisons which dont work well for weak coffee colored stuff. Soon realized the little dauber strips be the answer but I was fresh out wouldnt you know..kick kick. Thought for a bit of just dumping some sodium bicarb down in there but then I am tired of screwed up ferments so I dumped it on the ho made rock gizmos where the grass grows up through the cracks. That should teach it a lesson.
Some of the Aussie folks speak of putting some of the backset in the finished wash prior to running. Wonder has anybody ever tried that? I am officially scared to death of that stuff as regarding putting it in at the start of ferment. Hopefully this is some type figment phobia of an infertile imagination. Hey I just did a quick google on ideal ph dunder and got some hits to the no name yankee place going back to 2009. Person might could root some info out of there if they wanted lower the standards enough to visit. I did not notice any other likely looking sites.

Ideal PH is 4 to 4.5 before fermentation.

Tried the Aqua check strips got an Err PH was out of the test range. Read the manual online and range is pH: 6.1-8.8 Guessing I was below that. The soil PH tester reads 5-6... but not sure on the accuracy. So there ya go Big Wheel the lady that talked you out of buying one of those just saved you $50 or so!