How much detergent should I use based on my water hardness?

For soft water (0-3 grains), use 2 teaspoons in each cup; for medium-hard water (4-8 grains), use 5 teaspoons; for hard water (9-12 grains), use 8 teaspoons (completely full cups). For very hard water (over 12 grains), fill both cups completely and consider installing a water softener, adding 1 extra teaspoon per grain above 12. (Page 8)