OK. Here's another question about Brun Water: What difference is there is adding something like gypsum to the mash vs. adding it to the boil. For instance, with my current SNPA clone recipe, if I add gypsum to my water to get around 150ppm sulfate, my mash pH is predicted to be around 5.7. If I add more gypsum to get 300ppm sulfates, it drops my pH to 5.5. Both of these are OK mash pHs, but if I want that higher sulfate level is it more "efficient" to add the extra gypsum to the boil?
It may be more efficient to add it to the boil if adding it to the mash drives the pH down too far. However in most cases, it is best to add it to the mash. Calcium can be bound and precipitated in the mash reactions, but the more mobile ions such as Na, SO4, and Cl will predominantly stay in the wort and be carried into the kettle. Your sulfate contribution will make it to the kettle. The other good thing about adding minerals to the mashing and sparging water (and not adding them directly to the kettle) is that you are increasing the ionic strength of the water which helps reduce the extraction of things like tannins and silicates from the malts.
When the pH will be depressed farther than desired, then adding a bit of alkalinity could be the best way. In that case, use baking soda or lime. Lime is the preferred option since no sodium is added. However, last week during our discussion for the upcoming Water book, we came to the conclusion that using baking soda MIGHT be OK as long as the Na concentration is kept below 50 ppm. If you had no sodium in your starting water, 0.5 gram of baking soda per gallon raises the sodium to 36 ppm and the alkalinity rises to 80 ppm. That might be enough for many brewing situations. If your water already has a lot of sodium, then this option is probably out.