Converting Chlorine Dioxide concentration to Chlorine
You can, in fact, calculate the chlorine concentration from the concentration of chlorine dioxide. Chlorine will normally be in some compound, so when concentration of chlorine is given, it is usually concentration of chlorine compounds.

The concentration of chlorine dioxide is 0.6 mg/L. First convert the mg to g. this results in .006 g. Divide that value by the molar mass of chlorine dioxide (67.45 grams/mole) to get .00008895 moles of chlorine dioxide, or .00008895 moles of chlorine. Multiply this by the molar mass of chlorine (35.45 grams/mole) to get .003153 grams of chlorine. Convert this to milligrams to get 3 mg (one significant figure), so the concentration of chlorine from chlorine dioxide is 3 mg/L.