FWIW, BeerSmith's default pH model of "MPH 3" tends to be off (recommending double the optimal lactic acid additions, as noted elsewhere in this thread). If you switch to "BW" (which is supposed to be based on the Bru'N Water model), you will get (in my experience) far more representative results.
As an example, I built a grist of 10 lb. 2-row, 1 lb crystal 60, and 0.5 lb. crystal 120, with my (high carbonate load) tap water as the water. For the BW Model, BeerSmith calculated a mash pH of 6.03 for the BW model and recommended 7.6 mL of 88% lactic acid to get to a pH of 5.2. BeerSmith calculated a mash pH of 5.73 for the MPH 3 model, but recommended 11.7 mL of 88% lactic acid to hit 5.2. (I don't have "real-world" pH to report for this thought experiment). Based on past experience, I've found that the BW Model does better for my usual system.
If it's a HUGE worry, you're probably best switching to Bru'N Water, which really is excellent and has a ton of detail. That said, although I've played with Bru'N Water, I've stuck with BS3 for pH calculations: 1) it is one less software tool and one less step for me to worry about; 2) I don't care about pH *that* much (maybe I'll change my mind later); and 3) I've figured out my system to the point where I feel pretty comfortable in its behavior and my results.
I've also just noticed that MPH 4.2 has been created as a standalone spreadsheet, but I haven't played with that at all.'
Also, I ran across this post with a head-to-head comparison: https://www.brewginner.com/beersmith-vs-bru-n-water/