Fantastic Question. I've been trying to figure this out myself..
My theory (and it may be incorrect) is that I just try to get close to my "ideal" mash pH during the Mash with precise adjustment done pre-boil. A limitation of my "brewhouse" (I use Igloo coolers to mash + sparge with) is the inability to adjust temperature during the Mash. As a result, it's more important for my equipment + processes to ensure a stable temperature during the mash and nail pH after. With that being said, I do check the pH of my tap water and record my LA addition to Strike Water before brewing and log it for every single brew. That does help me make a very educated guess how much Lactic Acid to add to the Strike Water in order to get me close to my ideal mash pH (this is especially true if you brew the same recipe a few times). I check a pH @ 15 minutes into the mash, and at the end of the Mash. In my experience, I don't get much difference between 15 and 60 (or 90 minutes if I'm mashing very low for Belgians) minutes, especially with low lovibond malts. At most, a 0.1 pH change. Sometimes up to 0.3 with darker malts (I would probably check later with darker malts, i.e. in porters/stouts).
After I'm done with my Vorlauf, I always check my pre-boil gravity (be sure to adjust gravity for temperature at this stage- I use brewersfriend) and adjust to Goal Pre-Boil Gravity. I know my efficiency very well and if anything, I just have to add a little bit of Water. After the Gravity adjustment, I'll then take a pH reading and dial it in with LA to within 0.05 of my Goal (I use a MW102 pH meter, always calibrated at the start of the brew day). I shoot for pre-boil pH of 5.2 with light beers (Hefe, IPA, Belgian Blonde/Tripel) and up to 5.5 with darker beers (Porter/Stout). I usually shoot for somewhere in the middle if I'm brewing a beer with an SRM inbetween (i.e. Dubbel, Dunkelweizen)