The maximum allowed is calculated based on the maximum you can contribute as an employee ($19,500 + $6,500 for being over 50) plus the amount you can contribute as the employer (20% of self-employment income after half of the self-employment tax) minus your Roth 401(k) contribution.
To get to the maximum contribution which will maximize your tax deduction this year, you would contribute another $4,625 to your Solo 401(k). You can also opt to contribute that $4,625 to your Roth, but it would not yield any tax advantage this year.
