It's usually advantageous to first treat contributions as elective deferrals or Roth contributions up to the regular elective deferral (section 402(g)) limit.  This ensures that you are permitted to make the maximum permissible combined employee and employer contribution.

Once in the 401(k), there is no distinction between elective deferrals and employer contributions.  However, if your solo 401(k) allows and you choose to make Roth contributions, the Roth contributions can only come from the employee portion.  Employer contributions are always traditional, pre-tax contributions.

Note that your total contributions and your employer contribution will be separately limited by your net earnings from self-employment.  Net earnings are your net profit minus the deductible portion of your self-employment taxes.  The amount that you are eligible to contribute will be maximized by first treating all contributions as elective deferrals or Roth contributions, and any remainder (in your case there will be none) as employer contributions, and then, finally, as catch-up contributions if age 50 or over.  TurboTax determines your maximum permissible total contribution in this manner, so be sure to allow TurboTax to calculate your maximum for each category.  Your plan records should include the details of the split between employee and employer contributions to be able to substantiate that you have not exceeded the permissible limits.

In my own case, I allocate all contributions in the order detailed above because I contribute the maximum permissible under the law.

Thank you for the detailed and quick answer.

Seems two advantages for using the elective deferral first and the profit sharing later.
1. If one has a Roth provision, elective portion is the only way to use the Roth advantages.
2. It seems from the answer above that contributing first to elective portion helps maximize the available limit. Do net earnings depend on the amount contributed to the elective portion? If not, it is not clear to me why the total allowed limit will change with contributing first to the elective portion. Regardless, there seems to be no disadvantage to first using the elective portion.

Another advantage I can think of: Since exact profit sharing limit may not be calculated until actual tax filing, one can safely contribute the elective portion before Dec 31 (assuming income is well above the 18,000 IRS limit for 2017).

Follow up question: Seems I will be paying medicare tax on the solo-401k contribution regardless of whether it is classified as elective or profit sharing, is that right? Or can medicare tax be avoided in one of the classifications?

Regarding #2, section 415(c)(1)(B) limits one to contributing 100% of the participants compensation.  In this context, compensation is reduced by the amount of the employer contribution, but not by elective deferrals or Roth contributions, so by making elective deferrals or Roth contributions first, one maximizes the amount of compensation considered for the section 415(c)(1)(B) limit.

I can't imagine any disadvantage to allocating in the way that the calculation is done in Part III of TurboTax's Keogh, SEP and SIMPLE Contribution Worksheet.  This worksheet faithfully implements the IRS's worksheet in Chapter 5 of IRS Pub 560, used to calculate the maximum permissible deductible contribution.

Regarding the additional advantage you mentioned, yes, I considered including that in my answer.  However, if your net earnings are less than $18,000, you'll need to wait until you know your net earnings before your make the maximum permissible contribution.  In my own case, I do as you indicated.  I know well before the end of the year that I already have sufficient net earnings to support the maximum permissible elective deferral, so I make those contributions before the end of the year, but I wait until just after year end to make the employer contribution when I know the exact amount of net earnings.

Regarding your follow-up question, yes, you are correct.  The contributions, regardless of classification, do not reduce the amount on which self-employment taxes are calculated.

Thanks again!