You are down the hole and a little lost. Doctoring up the water, takes care of the water. It does nothing for the mash. Do not start saccharification until mash pH is established.
Malt will display an inherent pH, the malt data sheet will show the inherent pH. For standardising, the malster's use distilled water to determine inherent pH.
If you use RO water all by itself, mash pH will usually fall in the 5.7 range with standard brewers malt. Munich, Vienna, crystal, black malt are lower in pH than ale and pils/lager malt.
"Based on Bru'n Water, I'll proabably have to typically add some acid, and plan to use 88% Lactic acid. Do I just add it to the mash straight, or do I need to dilute it down a bit in some water?"
Use sour malt to adjust pH, you will be better off. Also, any other chemical which you dump in, might have to precipitate during the boil.
Right now, you have no sounding board to go from and you are assuming, that what someone is telling you to do, is the umbrella that covers all. It doesn't. There is absolutely no way that an on line calculator can advice a brewer on what chemicals need to be used, for the simple fact, the guy writing the program has no idea what is in the malt, which you are using on the brew day. The malt in the sack you are using today, may be different than the malt you use next time, as well. That is why the brewer needs to know what the inherent pH of the malt being used is, before dumping in acid. It is not a bad idea to learn about all of the data which is written on a malt data sheet. Otherwise, brewing beer becomes the same as throwing darts blind folded.
Crush a pound of malt that you use as the base and place it in an insulated thing large enough to hold the malt and the amount of water which you usually use per pound. Check pH of the brewing water and then heat it to 95F and dump it on the malt. After 10 minutes check the mash pH. Wait an hour and test pH again, if pH reduced, lacto did it. Take another pound of malt and do the samething, except raise the mash temperature to 125F, after 10 minutes test the mash pH. Wait an hour and test again. If pH reduced, the malt has phytase in it.
There is a part of the brewing process called the acid rest, it is there for a reason. After the rest is completed, only then, will the brewmaster adjust mash pH. After pH has been established, the mash temperature is increased to whatever the brewer decides to use as saccharification temperature.
If you decide to brew wheat beer and use a ferulic rest, mash pH has to be quite high, or the rest accomplishes nothing.
