For a sole proprietor, net profit is the amount on Schedule C line 31, which propagates to Schedule 1 line 3.
The only reason that you would not be eligible to make a nondeductible traditional IRA contribution is that you have no compensation to support the contribution. If you have no supporting compensation, you also are not eligible to make a Roth IRA contribution. If on Schedule 1 the sum of lines 15 and 16 equals line 3, you have no compensation to support any kind of IRA contribution and are not eligible to make any kind of IRA contribution.
