If the $100,000 is net profit, 20% of $100,000 as an employer contribution is a bit too high because of the amount that goes to self-employment taxes. You must first subtract the deductible portion of self-employment taxes, then multiply by 20%. The result works out to $18,587 of employer contribution (or slightly more if you max out Social Security tax due to also having another job). However, for 2018 the regular elective deferral limit is $18,500, so your maximum combined limit on Schedule 1 line 28 would be $37,087 (assuming no elective deferrals or Roth contributions to another employer's qualified retirement plan that would use up part of the $18,500). TurboTax calculates the maximum for you if you mark the Maximize box for an individual 401(k) (and you make no elective deferrals or Roth contributions to another employer's plan).
