TurboTax uses the Deduction Worksheet for the Self-Employed from IRS Pub 560 (called the Keogh, SEP and SIMPLE Contribution Worksheet in TurboTax) to calculate the maximum permissible contribution. The portion of the deductible amount in excess of the employee regular and catch-up elective deferrals would be the employer contribution.
Since you've already made the maximum permissible elective deferrals at your W-2 employer, you can only make an employer contribution to the solo 401(k). To allow TurboTax to calculate only the maximum permissible employer contribution you must select the Maximize box for either a Profit Sharing Keogh or a SEP contribution. The result will be the same as for an employer-only contribution to the solo 401(k).
You can view the calculations on the worksheet to see how the Maximum Allowed to Qualified Plans was figured.
Here's more info Calculating Contributions to Solo 401k.
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