Prior to adding detergent or chelant, the conductivity of water in a washing chamber is measured. The maximum concentration of hard water ions that could correspond to the measured conductivity is determined, i.e., it is assumed that all of the conductivity is from calcium and/or magnesium ions in the water even though other ions may in fact be contributing to the measured conductivity. Enough chelating agent is added to the chamber to sequester this maximum concentration of hard water ions and the conductivity is measured again. Using the two conductivity measurements, the actual concentration of hard water ions is determined. A chelant factor based on the actual concentration of hard water ions is then used to determine the amount of chelant to be added for subsequent wash cycles to sequester all of the hard water ions.