I find adding a teaspoon to an ounce or so of STPP to each wash load handles the mildly hard water here just fine. I use a bit more if the wash load is heavily soiled, such as work or gardening clothes, since STPP not only softens the water, but helps to break mineral based soils from fabric.
Since this is a Mediterranean climate, the tap water hardness varies through the seasons, harder towards the end of summer, softer after winter rains, and it gets harder after a period of prolonged drought. At least that's my impression.
For watering the lawn and garden, I use well water, which is VERY hard. However it's not potable, and the one time I tried using it to top off the fish pond, lost a number of fish. Oh well.