Adjustment of calcium, magnesium and alkalinity in drinking water?
The parallel thoughts:

The source water I am working with is:

12.6 mg/L or 0.126 mmol/L GH as CaCO3 equivalent
11.3 mg/L or 0.113 mmol/L KH as CaCO3 equivalent
  4.8 mg/L or 0.12 mmol/L Ca
  0.2 mg/L or 0.0082 mmol/L Mg
  1.5 mg/L or 0.065 mmol/L Na

And the desired target water is:

70    mg/L or 0.7 mmol/L GH
40    mg/L or 0.4 mmol/L KH
20    mg/L or 0.5 mmol/L Ca
  4.9 mg/L or 0.2 mmol/L Mg
  8    mg/L or  0.35 mmol/L Na

What is needed to add:

0.574 mmol/L GH, as Ca+Mg
0.287 mmol/L KH  as Ca+Mg as bicarbonates, I.e. 2x0.287=0.574 mmol/L of bicarbonate
0.38 mmol/L Ca
0.192 mmol/L Mg
0.285 mmol/L Na

Reaction of 0.285 mmol/L sodium bicarbonate
with 0.285 mmol/L lactic acid
releases 0.285 mmol/L carbon dioxide,
dissolving 0.285 mmol/L calcium carbonate
forming 0.285 mmol/L calcium bicarbonate.

( Not exact due equilibrium reactions but good enough )

What is needed to add:

0.287 mmol/L GH, i.e Ca+Mg    

0.093 mmol/L Ca
0.192 mmol/L Mg    

Adding 0.192 mmol/L MgSO4.

What is needed to add:

0.093 mmol/L GH, i.e Ca only  

0.093 mmol/L Ca    

This can to be added as 0.093 mmol/L calcium carbonate(optionally supported by pressured carbon dioxide) + 0.186 mmol/L lactic acid.