Edit: Title should say "100% vested" not "100% invested" lol

I'm 29 and only make about 50k a year at a commission based job tat pays an 8$/hour base pay. I'm expecting to make more as I get better at the job. Apparently I can put a percentage towards both pre-tax and post-tax. I'm planning on contributing a total of 8%, I think. Should all of the 8% go to the pre-tax or should I split it and put some in the post-tax roth?

"Safe harbor 401(k) plan: Effective January 1, 2018, this Plan is referred to as a safe harbor 401(k) plan. If your Employer elects to satisfy the "safe harbor" rules, then before the beginning of each Plan Year, you will be provided with a comprehensive notice of your rights and obligations under the Plan. However, if you become eligible to participate in the Plan after the beginning of the Plan Year, then the notice will be provided to you on or before the date you are eligible. A safe harbor 401(k) plan is a plan design where your Employer commits to making certain contributions described below. This commitment to make contributions enables your Employer to simplify the administration of the Plan by ensuring that nondiscrimination regulations are met, which is why it is called a "safe harbor" plan.

In order to maintain "safe harbor" status, your Employer will make a safe harbor matching contribution equal to 100% of your salary deferrals that do not exceed 4% of your compensation. This safe harbor matching contribution is 100% vested (see the Article in this SPD entitled "Vesting").

For purposes of calculating the safe harbor matching contribution, your compensation and deferrals will be determined on a payroll period basis.

Profit sharing contribution: Each year, your Employer may make a discretionary profit sharing contribution to the Plan. Your share of any contribution is determined below.

Your share of the contribution: The profit sharing contribution will be "allocated" or divided among Participants eligible to share in the contribution for the Plan Year.

Your share of the profit sharing contribution is determined by the following fraction:

For example: Suppose the profit sharing contribution for the Plan Year is $20,000. Employee A's compensation for the Plan Year is $25,000. The total compensation of all Participants eligible to share, including Employee A, is $250,000. Employee A's share will be:

$20,000 X $25,000 divided by $250,000, or $2,000."