RA is not a targeted value. It is a major influence on mash pH, but RA isn't the target, pH is. Adjust calcium, and magnesium concentrations as desired to meet beer flavor goals and adjust bicarbonate to achieve mash pH target. Those three components are the basis of what we calculate as RA. It is because we have to vary the bicarbonate to match the requirements of the grist, that we can't target a specific RA.