I reverse engineered my formula to see how well it works for New York City.
Population 8,000,000 x 30% = 2,400,000
2,400,000 divided by 2 = 1,200,000 divided by 10 = 120,000
So 120,000 confirmed cases needed to reach 30% immunity threshold
120,000 x 10 (asymptomatics) x2 (T cell immunes) = 2,400,000 = 30% of population
Then I looked for date New York City reached 120,000 confirmed cases.
April 15
According to the graph, cases peaked from April 5 to April 12, then started to decline rapidly.