How much detergent should I use for hard water versus soft water?

For soft water (0–3 grains hardness), use a minimum of 1 tablespoon in the main cup for POTSCRUBBER and Normal Wash cycles. For hard water (7–12 grains), fill both the main cup and open cup completely, noting that the filled main cup holds 3 tablespoons and the filled open cup holds 2 tablespoons. Water with 12 grains or more is extremely hard and a water softener is recommended, as lime can build up in the water valve and may cause flooding. (Page 7)