"Ben" in the IRS example is not self-employed, Ben is an employee of an S corp. You said that your wife is self-employed, implying that her LLC has not made an election to be an S corp. If your wife has made the election to be an S corp, contributions to the solo 401(k) are handled on the S corp's tax return and any elective deferral would have been reported in box 12 of her W-2 from the S corp.
Assuming that your wife is self-employed (the LLC is not an S corp), see the worksheet in Chapter 5 of IRS Pub 560 for the method used to calculate that the maximum permissible contribution to the solo 401(k) is $10,687 due to being limited to net earnings (section 415(c)(1)(B) of the tax code). Net earnings are net profit minus the deductible portion of self-employment taxes.
