OK, first, I am not good at using spread sheets for water. I referenced from Al Korzonas's Home Brewing. Old book, but has some useful charts. I know my water has 12ppm sodium. London water has 80-100ppm. 1 tsp/5 gals gives 110ppm. Therefore, 1/2 tsp yields 55ppm, added to 12 = 67ppm, still...