Calculator for COVID-19 population immunity, likely optimistic

I wrote a simple spreadsheet to estimate the approximate level of immunity for a given area. The inputs are the 

• population,
• number vaccinated,
• number of COVID-19 cases.

The key output is the fraction of the population 18-years-and-older with some level of immunity, either from having been sick or from having at least one vaccination. 

Here's the spreadsheet:  US percent immunity.

Looks like very good news to me! The U.S. as a whole appears to be at about 80% as of this writing.