Deductible contributions to the solo 401(k) reduce the amount of compensation left to support an IRA contribution. As you suspect, the sum of the deductible portion of self-employment taxes, the self-employed retirement deduction and the IRA contribution are not permitted to exceed your net profit from self-employment (absent any compensation from a W-2).
Because only deductible contributions to the solo 401(k) reduce compensation available to support an IRA contribution, you could make some amount of your your employee contribution to the solo 401(k) be a Roth contribution (plan permitting) allowing you to use the same compensation to support an IRA contribution. (A quirk in the tax code.) TurboTax automatically determines the amount of compensation available to support an IRA contribution. Note that unlike an IRA, there is no provision to recharacterize 401(k) contributions between traditional and Roth, so if you have already contributed the maximum elective deferral, you won't be able to change that to Roth.
