I have to brew with RO water since I live in the land of municipally ion-exchange softened water. I can't use the tap water. My RO water quality is fairly similar to the OP's water quality above.
I find that adding alkalinity is frequently needed when the crystal and/or roast malt content is more than several percent of the grist. Having a pH meter is a nice double check and its very necessary for me since I continue to gather data for the mash acidity/pH model used in Bru'n Water.
I find that the latest acidity/pH model is now quite accurate in practical usage. That model has not been added to the existing version 1.10 that is in wide use now. It will be added to the next version.
To reinforce the validity of the model, I'll relate my last brewing session to all. I brewed an English Mild that contained 8% of an English dark crystal malt. My LHBS had Simpsons Dark Crystal in a bin and the placard said it was 77L color, but someone had marked through the color rating and written 155L in Sharpie on the bin. I tasted the malt and loved it, so I was going to use that malt regardless of the color. It turns out that Simpson also makes an Extra Dark Crystal with a 155L rating. Nobody at the shop knew if this was Dark or Extra Dark in the bin.
Overdosing your mash with pickling lime is definitely not a good idea. A lower than desirable mash pH is better than higher, so I knew that I could not assume that I had the extra dark crystal. I'd have to risk underdosing the pickling lime and add more later by assuming the 77L color rating. Bru'n Water said I needed 0.6 grams of lime in the 2.5 gallons of mash water. The resulting mash pH was measured at 5.2. That was well below the 5.5 pH that I was aiming for.
Unfortunately, I did not think about checking (before the brew session) what the lime addition would have to be if the crystal was actually 155L. So, I was flying blind with regard to how much more lime I needed in the rush of the mash. I added another 0.2 grams and the pH came up to 5.3. That was not good enough for me, so I added another 0.2 grams and the pH rose to 5.4. Good enough.
After the brew session, I had a chance to sit down with Bru'n Water and see what the predicted pH was if 155L was used as the crystal's color rating. With that color rating change and the added 0.4 grams (1 gram total), Bru'n Water did predict a mash pH of 5.4. I was surprised and relieved that the model is still pretty accurate and reliable.
The more I work with the model, the more confident I am that I can get within 0.1 pH unit of actual every time. I have pretty high control and monitoring of my raw ingredients and my results may not be typical. Therefore I'm not willing to claim that result for everyone. But its pretty safe to say that the predicted pH will be within 0.2 units every time. A brewer's water or ingredients would have to be way off to produce an error greater than that (see my result above).
The bottom line is that if you are confident in your water report and the ingredients used, when Bru'n Water says you need a certain addition, do it. Its far more likely to be right than wrong.
