Interesting. Usually hard water, as it relates to water softening is expressed as grains per gallon and that's how my water softener is set for recharge. Reverse Osmosis units though go by PPM of solids in the water. My RO has a sensor attached and when the PPM reaches a certain point it turns a red light on.

Yes I'm very fond of soft water. better for appliances, pipes, showering and shaving. I use KCl insterad of NaCl