On Line 18, you're asked to make an estimation of the portion of your net profit or loss, reported on Line 3 of Schedule SE, that you made between March 27, 2020 and December 31, 2020.
The instructions for Line 18 in the IRS's 2020 Instructions for Schedule SE are very vague, but also open-ended, allowing you to use any reasonable method:
Use any reasonable method to divide the net profit or (loss) you reported on line 3 between (a) January 1, 2020, through March 26, 2020, and (b) March 27, 2020, through December 31, 2020. A reasonable method will accurately reflect the time when income was earned or accrued and deductible expenses were paid or incurred, depending on your method of accounting. In most cases, a proportional division of the amount reported on line 3 based upon the number of days in each period will be considered reasonable.
