The key is the difference in gH between pre- and post- treated water. To calculate the amount of sodium / potassium added by the water softener based on the gH drop: How Much Sodium Does a Water Softener Put into Your Water? ?[censored]Pure Water Products, LLC
In my case, a 4 degree drop (from 8 to 4 gH) was adding too much P and we turned the softener off via a by-pass valve.
Another consideration is that the efficiency degrades over time due to the removable and coated resin satturation. The removable resin can be replaced yearly (~$80). Re-coating the tank is a much more expensive proposition. If your system is rated at 4,000 gl per cycle new (before back flash), you need to re-coat when the system degrades to 2,000 gl per cycle. The side affect is that the amount of salt / potassium added to the tank decreases and the residual Ca and Mg increases over time.