I am fixing my return how do I calculate what number to put into Sch SE-T: Max deferral line 18 on my self employed return ?

The instructions for Line 18 are very vague, but also open-ended, allowing you to use any reasonable method to calculate the amount:

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.

Please see the instructions for Line 18 of Schedule SE in the IRS's 2020 Instructions for Schedule SE for more guidance.