I just finally got an answer back from my local city water utility.
Quote: "The average hardness last month was 7.2 grains. Hardness varies throughout the year from 7 to 10 grains/gallon"
When asked about total dissolved solids - Quote: "We don’t test for Total Dissolved Solids so I don’t have any values to provide you for that." (insert eye roll here)
I guess I'm mixing RO/Rain/distilled water with my tap water. The question is at what ratio.