This was the tip I needed to get the portion of IRS.gov that refers to this. For everyone else see:
Deferral of employment tax deposits and payments through December 31, 2020
See # 24
How can a self-employed individual determine 50 percent of the Social Security portion of self-employment tax attributable to net earnings from self-employment earned during March 27, 2020 through December 31, 2020? (added July 30, 2020)
Self-employed individuals may use any reasonable method to allocate 50 percent of the Social Security portion of self-employment tax attributable to net earnings from self-employment earned during March 27, 2020, through December 31, 2020. For example, an individual may allocate 22.5% of the individual's annual earnings from self-employment to the period from January 1, 2020, through March 26, 2020, and 77.5% of the individual's annual earnings to the period from March 27, 2020, through December 31, 2020. Similarly, an individual may use any reasonable method in applying the Social Security wage base or taking into account partnership income in determining the portion of 50 percent of the Social Security portion of self-employment tax attributable to net earnings from self-employment for the period from March 27, 2020, through December 31, 2020.
