The math looks OK but you probably wouldn't need that much. It would be rare for a treatment plant to go the whole 3 mg/L with chloramine and not all plants go to 3 mg/L (it can be a seasonal thing) total. If the water works does go the max 3 mg/L and half is Cl2 you can just let it stand overnight and that half is gone meaning you could use half of 713 mg. The bottom line is that you can use as little as it takes to get the smell to go away (agitate the water to bring any odor up as you sniff). Or you can by total/free chlorine test kits from aquarium stores, Hach, LaMotte and others which will tell you how much chlorine and chloramine are present. Knowing that chloramine takes twice as much as chlorine you can figure out how much total metabite is needed. Much simpler to add incrementally and sniff, IMO.