I'm not so sure that mash pH will translate to final pH in this case. Meaning I doubt that identical Flanders, with the only difference being 5.2 and 5.4 mash pH, would result in final pH s of, for example, 3.2 and 3.4. On the other hand, it could also be that the 5.4 might go lower than the 5.2. Someone should do an experiment on how mash pH effects final pH in sour beers.
My personal method is to make wort as normal. The boil is the same except for less hops. And with this year's sour beers, because I now have a good meter, I will be adjusting my wort post boil, pre pitch, to 4.5. Reason being that I pitch lacto first for a week at 90 and I think that manually lowering my ph to 4.5 will inhibit anything that survived the boil or drifted in between the boil and the pitch.