Is there a water hardness code?

I just got my parents used Rain Soft Gold series softener who were on city water. I hooked it up at my house on city well water that is very hard. It is working and softening the water but I need to know how to set it for much harder water than it was originally set up for. My city web site said to start with water softeners set at 18 grains and go from there. I've read on this site that I can access these type of settings by pushing and holding the rain drop in the corner of the digital display. Can anyone tell me what to change once I do this?