I was worried about impurities. So I checked the KOH/NaOH bottles. The KOH says > 87% (kinda low!) but the NaOH says >99.7% and both are behaving somewhat similarly.
I prefer the formula you posted both for reasons of simplicity and economy but I need to sort the pH thing out. I'll see if I can calculate expected pH theoretically. Something that just crossed my mind is that I should also stick the pH meter into a couple of bottles of commercial Rodinal that I have.

But more fun will be to actually test the developer on film and compare results. My worry (other than lack of understanding on my part) is that higher pH may lead to higher grain.