The main mash step is complete and "Water Additions" in Bru'n Water entirely emptied ready for next mash step thanks to having shifted the main mash mineral additions to the "Water Report Input". Go to the "Grain Bill Input" page and temporarily answer "No" to the "Remove
Roast Malts from the Main Mash" option. The "Estimated Room-Temperature Mash pH" indicator falls through the fall (it shows pH 5.06 on mine). It's why holding back large amounts of roast grains from the main mash is a good idea.
There is still undamaged starch in these grains that could be mashed, especially this Chocolate Malt where roasting hasn't had as much time to damage the starch. And there are plenty of enzymes left in the main mash to do this (but much of the
beta-amylase will have "burnt-out" so this is a dextrin recovery exercise, not a maltose one).
In actual practice you'd start heating the mash to 70°C (a recirculating mash system, HERMS or RIMS, will be handy for this), stir in the roast grains, and prepare to add the extra mineral salts. If you are only planning to steep grains (maybe you've chosen to add crystal malts?) you may choose to heat to "mash-out" temperatures (75°C?).
When planning mineral salts to add, start with the "Hardness" salts (Calcium and Magnesium) because they will impact pH. Don't be too precise yet because you may have to juggle with some amounts (especially chloride salts). I succeeded just adding Epsom salt (magnesium sulphate) and calcium chloride (Magnesium salt considered first). The pH fell even further ... 4.67! That's why I said Hardness salts first!
Now to get the pH prediction back up. Say to 5.3? Alpha- amylase is much happier in this, more acid, environment than beta-amylase, but don't go mad! Start adding (virtually ... in the calculator) Sodium Bicarbonate (Baking Soda) to get the estimated pH up. Baking Soda is good because it's fairly quick when it is actually altering pH. Lime is very sensitive to having the dose just right and is harmful to you if you're not careful with it. Chalk is too slow for these "on-the-fly" changes.
Add salt (NaCl) last to get the Sodium level up to the chosen "profile".

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Record the alkalinity salt addition details in the "Batch Notes" before: Zero the Alkalinity salt quantity, set the "Add Hardness Minerals to Kettle" to "Yes" (anyway, we're not adding them to the Kettle, they go in the "second" mash) and on the "Grain Bill Input" page set the "Remove
Roast Malts from the Main Mash" option back to "Yes". Everything is then back for "first mash" again. Remember these
three salt operations.

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With "Add Hardness Minerals to Kettle?" enabled the affected entries only change colour (orange background) to indicate that are removed from the main mash and included in the boil. "Salt" isn't removed, but in this case would be better if it was. Optionally add the two "salt" amounts together (physically) and put in boil as it evens out the figures so they don't seem "suspicious".
I don't entirely agree with copying geographic "water profiles" into a water calculator, but it's a good game trying to resolve them. This example here was resolved to within 15ppm of Chloride ions, but the target was 341ppm for Chloride so 15ppm discrepancy was less than 5% (of chlorides ... all other ions counted matched their targets exactly). The highly mineralised profile was picked to purposely make the exercise difficult. The "Normalization" process used in the exercise will remove a lot of Calcium from the finished mashing profile, but as yet I've not attempted to reduce the Calcium over the entire profile. Perhaps I should?