Out of curiosity, do you measure the pH of your culture and doughs and, if so, what values do you look for?
Yes, I do, and use pH to establish three basic parameters for any dough, including my maintenance starter, dough starter and final doughs. The first is to establish a standard baseline after the initial mix, which varies per dough, quite obviously, based upon the amount of flour prefermented. I use this to ensure, first, that the correct pH was obtained for the pre-dough going into the mix (the starter) and also to check to see mixes were correct (in this scenario I use it less often). Second, I use it to establish the final pH for doughs, in particular starters, which I never want to go below 4.0 / 3.9 before refreshment, with my maintenance starter in particular. (This is because I am aiming to maintain a particular culture.) Lastly, I establish a pH that, when coupled with FDT out of mix, dough temperature during fermentation and time spent in bulk, will let me know when it's time for dough division; again, as before, this number varies for each dough but rarely does not go below 4.5 / 4.4, when "enhanced proteolysis" occurs.
If used for a predictive model, the results become more complicated, though. pH means nothing without such considerations as temperature, time and substrate (ash content). Here, theoretically-predicted data would need to be reinforced with real-world statistical data, especially hard since most American flours do not adequately list ash content of their flours, fermentation temperatures tend not to be constant, and so on.
I would also be interested in knowing what Peter asked since I am doing pH experiments. I also wonder if it matters what the pH of the water that is fed to the starter and also about what kind of flour is best to be used to feed cultures.
pH of water matters less than you might imagine for starter cultures; starters prefer doughs that are more alkaline than acidic to begin with. The pH of water can have other effects on the quality of a dough out of initial mix, quite obviously, but I do not think that's the question being asked.
As per flour, there's one basic rule of thumb, especially for maintenance starters: the more whole grain and fresher it is, the better. Lactic-acid bacteria, Lb SF
in particular, evolved to ferment and utilise every fraction of the wheat, rye, spelt and/or teff kernel as part of its metabolic life-cycle. Flours of lower extraction favour yeast fermentation due to greater sugar content and because of their low buffering capacity. This is where grain mills come in handy. Using fresh, whole-grain flours has a noticeable difference on nutrition, flavour, rheology, a culture's activity, etc.