I realize this is an old thread but I have a few questions on this topic. I have been using Bruunwater for several years and my beer is definitely better for it. My source water has 284 ppm bicarbonates so I've had to make significant adjustments to it. I cut it with distilled water and often start with >50% distilled water. I usually use CaCl, CaSo4, and lactic acid to adjust my water and really don't ever go above 1 ml/gallon of finished beer with the lactic acid for fear of an unwanted flavor contribution. I recently picked up a bottle of 10% phosphoric acid at my HBS. It is my understanding that phosphoric acid has no/limited flavors so that it could be used in greater quantities. I realize that 10% is a fairly weak solution and would require more to be effective. I just ran the numbers on a beer (2B) that I want to brew tomorrow (42% pils, 42% vienna, 14% corn, and 2% midnight wheat). I used lactic acid to correct the mash. I cut the sparge with 50% distilled water but still need to add 33ml of phosphoric acid to bring the sparge pH down to 5.5. My question is whether this is a good idea- seems like a lot.
Hopefully Martin will see this and add his recommendations, but I can tell you that my tap water's bicarbonate is 151 ppm, roughly half of what yours is. Anyway, I use 85% phos acid dosed via syringe prior to heating my strike water and I always come in below the taste threshold (in the final beer) using 100% tap water for most ales. My thought is that would be similar to you using 50% distilled or RO water + phos acid. Get yourself some higher % phos acid. You can google for 85% strength - it's sold by such businesses as Duda Diesel, and normally available via Amazon. When I make pilsner I cut my water with 50 - 75% RO, so I would recommend you go with 100% RO for pilsners.
My other salts etc. added to mash water before stirring in the grist are gypsum, calcium chloride, Epsom salt, canning salt, and sometimes for dark beers pickling lime (added directly to the mash)
I hope that helps.
Edit: I forgot to mention that with the method mentioned above, sometimes I come as close as 70 - 90% of the level established as still under the taste threshold for phos acid - as I've previously read by knowledgeable folks. I'm just too lazy right now to go back and research what that threshold is.