The calculators are a handy tool! I have them but I don't use them much now since I let my experience with water treatment guide me. I know we all love to "nail the numbers", but after knowing and dialing in your system you need to put your experience to work. Adding acid to your brewing water is simple if you have the right tools.
I treat the total water (mash & sparge) with 10 % Phosphoric Acid. My beer has improved significantly! My rationale here is if it works for
Sierra Nevada, it can only make my beer better (and it has).
I use two pH meters, a Hanna Piccolo Hi 1280 & a Martini pH55. I make sure my meter's electrodes are conditioned by soaking in fresh storage solution 24 hours before I brew. I use a couple of tall double shot tequila shot glasses to soak. Tip.....The Hanna unit stays wet in storage better than the Martini, so if you own a Martini you need to add drops of storage solution weekly. I calibrate usually once a month. Building from RO water, I determine my sulfate/chloride ratio and add minerals (avoiding chalk since it is a poor dissolver) to room temp water prior to adding acid. Always starting with 28 gallons, I know I can add at least 20 ml of 10% phosphoric acid, recirculate the water, measure the pH, add a small final addition of acid to get the water to about 5.4 pH. My mash is usually about 5.3 pH. My porter was 5.2 pH. I haven't seen any drastic differences between dark & light malts to loose sleep over. Soak your meter in cleaning solution for 5 minute after EVERY brew session. Learning to take care of your meter is essential! My meters are in close agreement and they function to double check each other. They are both good for brewing!