You are correct, TurboTax is unable to take into account deferrals you made to other employer's plans, so you can't use the Maximize function for an individual 401(k). Since you've already done the maximum elective deferral, you can cause TurboTax to calculate just the permissible employer contribution to the self-employed 401(k) and put only this on Schedule 1 by using the Maximize function for a SEP contribution instead.
