I would research your specific boiler to determine the water treatment requirements. I am a licensed Boiler Technician, and many of the boiler manufacturers literature is poorly written. I can only guess someone wrote a manual badly 40 years ago and many boiler companies simply copied the original bad one. Some boiler manuals poorly state that water should be maintained at below 3 grains per gallon, most modern manuals now say 3 ppm. There is also many other considerations. Is your boiler recieiving chemical treatment? Have they done an LSI for your water and pH? Does your boiler manual recommend that you bypass the softener annually for a week to add a thin protective layer of calcium to the tubes? etc... Many boiler technicians read a manual once, and assume it refers to all boilers, most boilers have completely different needs and water requirements. For the most part, we try to maintain below 3 ppm of hardness for most boilers, but... get the manual and start reading, and follow the manufacturers recommendations.