TurboTax's Maximize function for an individual 401(k) doesn't take into account elective deferrals or Roth contributions you've made to another employer's plan, so you have to use a workaround. In the individual 401(k) section you can enter up to a $14,500 elective deferral or Roth 401(k) contribution, but to calculate the maximum employer contribution you'll have to use the Maximize function for a SEP contribution. The employer-contribution calculation is the same for both a SEP plan and an individual 401(k).
With $30,000 of net profit from self-employment, your maximum employer contribution to the individual 401(k) is $5,576. Adding the $14,500 of elective deferrals brings the total maximum contribution to your individual 401(k) to $20,076 (or slightly more if you max out the Social Security wage cap between your two jobs, but no more than $20,420).
