I recently had a question from a Bru'n Water user that has the fortune of brewing with tap water of near Pilsen quality. He was trying to create a better Pale Ale since his prior attempts lacked 'zing'. So, he was trying to match the calcium content of the Pale Ale profile in Bru'n Water (140ppm). He did not realize that for calcium, anything over 50 ppm is good enough. The only reason that the calcium content on the Pale Ale profile is high is because you have to add a lot of gypsum to deliver the high sulfate that you want.
If you try and take the Ca to 140 ppm in water with very low alkalinity, the mash pH is likely to be far lower than desirable. I just did a quick test in Bru'n Water using distilled water as the starting water. I added epsom salt until the Mg was high enough, then added gypsum until the sulfate was high enough. Then I added table salt until the sodium was high enough, then added CaCl until the chloride was high enough. That left me with all the ions excepting calcium near their targets (Ca was at 115 ppm). That produced a 5.3 pH with a 90% Pale malt and 10% crystal 40 grist. Its a little low but not terribly so. If I wanted, I could cut back on the table salt and add baking soda to produce the intended Na content to produce a 5.4 pH. Either is workable.
Another option for avoiding an excessively low mash pH is to reserve the Ca and Mg containing minerals from the mash and adding them directly to the kettle. That avoids the low pH issue in the mash. As many of you probably know, those hardness minerals combine with phosphate compounds in the mash to reduce pH. But since those phosphate compounds are also transferred over into the kettle, I figured that this pH lowering effect would still occur in the kettle. But I was not positive of this, so I posed the question of what the effect of adding those minerals directly to the kettle was to my partners for the upcoming Water book: Palmer, Kaminski, and Delange. They confirmed that the effect would occur in the kettle.
You may now be asking...so what if the pH in the kettle is driven lower? Well, a lower than desirable kettle pH also affects several factors in the beer. The number one factor is that low pH reduces the production of hop bitterness and hop expression. The more ideal wort pH of 5.2 to 5.6 helps extract the hop alpha acids and other components from the hop matter. The rest of the factors are not a big deal, but the hop thing is. So getting the wort pH right is a good thing. Therefore, maybe the technique of reserving the hardness minerals from the mash is not ideal. Getting the pH correct in the mash with all the minerals is probably a more ideal way to go.