RA = alk - (Ca_concentration - Mg_concentration/2)/3.5.
RA, alk and the concentrations are all in the units of milliequivalents per liter which are the units in which the concentrations of acids are specified. When alkalinity is measured acid is added to the water sample until a certain pH is reached.
(Ca_concentration - Mg_concentration/2)/3.5 represents an effective acid addition. This is the acid (protons or hydrogen ions if you prefer) released when calcium and magnesium react with malt phosphate. This 'acid' offsets (neutralizes) the alkalinity (bicarbonate ion) in the water converting it to CO2.
Acid from any other source i.e. acid from a bottle or acid from a dark malt or from acidulated malt has the same effect. It neutralizes water alkalinity. But this is not sufficient in most cases - certainly not in light beers as base (pale) malts have an alkalinity of their own which must be neutralized. Thus you must supply enough acid to overcome the water's alkalinity and the malt's alkalinity. You must add dark malt or acid or acidulated malt or a combination of these.
Note: In North America in water and brewing practice mEq/L are often multiplied by 50 when talking about alkalinity, residual alkalinity and Ca/Mg ion concentrations. The resulting units are 'ppm as CaCO3' because if 100 mg/L chalk (CaCO3) is dissolved in 1L of water using CO2 gas, as nature does things in the ground, the resulting water will have alkalinity and hardness of 100 ppm as CaCO3.
