Resolution will be based on the popular vote total for the office of the presidency from the Federal Election Commission report that comes out after the election.
Numbers for the 2008, 2012, 2016, and 2020 elections are based on the FEC reports that followed those elections:
2020: https://www.fec.gov/resources/cms-content/documents/federalelections2020.pdf
2016: https://www.fec.gov/resources/cms-content/documents/federalelections2016.pdf
2012: https://www.fec.gov/resources/cms-content/documents/federalelections2012.pdf
2008: https://www.fec.gov/resources/cms-content/documents/federalelections2008.pdf
2020 was an absolutely historic year for turnout by American standards. It was the highest percent since the South clamped down on voting restrictions around 1900. And the voting eligible population is growing as slowly as ever.