The issue with the BeerSmith pH estimator is that it only works with the water profile tool. Adding mineral salts, acidulated malts, acids, etc. to the recipe does not get taken into account when calculating the mash pH. To get it to work, you need to use the water tool, make your adjustments, save it as a target profile and then add that water to the recipe. Any further changes you make once you add the water will not be taken into account in the pH calculation. So you need to delete the water and the mineral additions, go back to the water tool and do it all over again. Yeah, this can get awkward at times.
I agree with KellerBrauer in that the acid recommendations and acidulated malt recommendations seem to be overstated by at least 50%.
Once applied though, it seems to slightly overestimate the pH (using mineral additions only) by slightly less than .05 units. Bru'n Water seems to continually be understating the pH (at least in my system, my water, and the malts I use) by almost the same value. Since there are some considerable differences between malts and maltsters which both programs must make assumptions about, this seems to be a reasonable margin of error.