Because you've maxed out the employee elective deferral at your W-2 employer, you can't use the Maximize option for a solo 401(k) contribution because the Maximize function for a 401(k) contribution assumes no employee contributions to any other 401(k), 403(b) or SIMPLE plan. As you observed, the Maximize function is incapable of taking into account amounts reported in box 12 of a W-2. You'll instead need to use the maximize option for a SEP-IRA contribution to calculate just the permissible employer contribution to the solo 401(k).
