$12,500 of net profit minus the deductible portion of self-employment taxes, $883, leaves $11,617 to contribute to retirement accounts, assuming no other earnings from another employer. Having already contributed $8,500 of the $11,617, that leaves $3,117 available to cover additional contributions.
I don't see why you would not want to maximize your traditional IRA or Roth IRA contributions before contributing what remains to the individual 401(k). The IRAs are generally more flexible, particularly with regard to accessing the funds before age 59½.
