Deferral of self-employment tax payments

To get the deferral to flow to Sch 3, you need to make the election to defer.  The input fields are under Taxes > Other Taxes > Self-Employment Tax (Schedule SE), in the Deferral for Certain Schedule SE or Schedule H Filer section.

In terms of the percentage, you have misread what the instruction says.  77.5% is not the maximim but an example.  In fact, the instructions as well as the guidance published on the IRS' website state the following [emphasis added]"

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.

Most self-employed individuals use the cash method of accounting and will therefore include all income actually or constructively received during the period and all deductions actually paid during the period when determining their net income from self-employment.

Assuming your client's income and expenses are received and paid ratably over the year, the proration will be the default PTO uses, i.e. 280/366 = 76.5%.

In case your client uses a different method to determine the net income for the period 3/27/20 to 12/31/20, you can override the default calculations on the same input screen.