Beginner water chemistry question

[Maegnar]

Hello fellow brewers!
I'm not sure this topic should go into the beginners forum, but I'm only starting to play with water treatment and need a bit of help.
So I'm planning to brew a Sweet Stout tonight and I'm trying to calculate my salts and acid additions.
Here's the calculation from brewer's friend:
http://www.brewersfriend.com/mash-chemistry-and-brewing-water-calculator/?id=5TPXDZC

The problem in short is following: my water has pretty high HCO levels - 238 mg/l and this seems fine for dark and malty sweet stout. But if I add acid to hit 5.4 pH, HCO drops to 88.7 mg/l. So what is the correct move here? Should I procede with HCO = 88.7 mg/l, because only the base HCO matters? Should I skip acid addition and go with 5.52 pH? Should I add chalk and baking soda and add sufficient acid to hit both HCO and pH targets? (in last option I will be addind 8g of each baking soda and chalk and my Ca and Na will hit through the roof)

Thanks for the help and sorry for my slightly advanced-beginner question if it's in the wrong sub

 

[McKnuckle]

Here's what I'd do. Get rid of the table salt. Get rid of the phosphoric acid completely. Up the CaCl to 2.4 ppm.

Uncheck "salts added to mash only."

You now have a 5.51 pH, perfect for stouts, and appropriate mineral levels for the profile you're trying to match using a MINIMUM of additives. Literally matching the profile is not important, and is - in my opinion - a big source of headaches that some of the water software causes for people.

The native 21 ppm of Na in your water will still "round the corners" of your stout, but it won't risk being perceptible AS salt the way 40 ppm might.

Do not pay any attention at all to HCO as a figure to match. You care only about the pH. (Say this to yourself out loud until you believe it. )

 

[Maegnar]

So basically my 2nd option, minus the table salt. To clarify on my table salt addition - I was aiming for 2:1 Cl:SO4 ratio.
Thank you very much McKnuckle! Just on the side note - would you care to elaborate why should I ignore HCO? I'm not doubting you, but I like to know at least the basic reasoning behind one or other options.

 

[McKnuckle]

You ignore HCO because it's not a flavor or character component; it's indicative of the residual alkalinity of the water, which affects the all-important mash pH. It's not something that is adjusted in its own right to attain some end-game characteristic.

 

[Maegnar]

Somehow I've thought of it is higher for dark beers and lower for light ones. so since this was a stout, I was worried something would go off with lower HCO.
But yes, it makes sense I should ignore it, since I will have appropriate pH and meaning the beer should not come out sour-ish, as starting HCO has balanced off the roasted malts addition and therefore became lower.