Deductions & credits

Neither.  Both of those line items can and usually do contain income not considered compensation.    If all of your income comes from self-employment 1099s, your taxable compensation is your business net profit,  Schedule 1, Line 3 less self-employment tax deduction which is on Line 15 of Schedule 1.    TurboTax can calculate your maximum contribution for you.  

Once your self-employment income is entered:

1. In your return, type SEP IRA contributions (use the whole phrase) in the Search box.
2. Select the Jump to link at the top of the search results.
3. This will take you to the Self-Employed Retirement Plans screen.
   • If you get a message that Self-Employed Retirement Plans are only for self-employed individuals, this means that you haven't entered any self-employed income.
4. TurboTax will ask you whether you contributed to different types of retirement plans. Select Yes to Solo or Roth 401k.
5. Check the box to Maximize Contribution to Individual 401k.   
6. Click Continue.