Fluoride levels in the Great Lakes range from 0.05 to 0.14 mg/L (Anon, 1977) 

https://www2.gov.bc.ca/assets/gov/environment/air-land-water/water/waterquality/water-quality-guidelines/approved-wqgs/fluoride-tech.pdf

.1*(3*1.774 + 1.4 + .5)/4 = 0.18055

so, i think that's a reasonable background assumption until i get further clarification.