Resolves as a percentage equal to the voting-eligible population turn out rate (total ballots counted).

Market closes on election day, resolves when the first estimate becomes available from a credible source (my criteria, open to discussion).

 publishes an estimate. For reference they have 66.6% for the 2020 so this market would have resolved with 67% for the 2020 election.

If a month after the election there is no estimate I will calculate it using as numerator an estimate of total voters (again from I believe is a credible source) and using as denominator 258,676,000 (which is my estimate of voting-eligible-population).