Slaked Lime to raise pH

BACKGROUND

I brewed a tripel recently with 100% RO have noticed that Castle pilsner may have a lower pH in distilled water than other pilsner malt. I did a test mash and determined I needed less acid than predicted to achieve an actual mash pH >5.3 at room temperature.

I want to brew a Dark Strong with >65% Castle pilsner and other darker grains and did a test mash last night while brewing another beer because I suspected that combination may result in a pH <5.3 without any acid and 100% RO water. That indeed was the case and was able to fine-tune the pH prediction based on the test to develope a plan to brew this. I can either use 25% of my water as tap water [350 ppm alkaline (CaCO3)] and no acid or use 100% RO with slaked lime.

QUESTION

I generally treat all my water for brewing the same but never used a base before. With bases, would you add them to the mash only? If I mixed water, would I mix tap water in for the mash only and use 100% RO water for sparging? I batch sparge.

I've only brewed ligher colored beer ever since starting 100% RO and have never saw the pH go lower than could be corrected by using less acid malt.

Yes when your water has too little alkalinity, adding a base or basic buffer may be necessary for mashing water. But, its never 'needed' in sparging water. Low alkalinity is a desirable trait in sparging water under all circumstances.