it seems this same question has been asked over and over. anyway. the only thing you need to do in TT is check maximize which you already have done. there is a worksheet in forms mode that will show you the maximum you can contribute.
see this for maximum amounts
https://www.fool.com/retirement/2017/12/09/the-maximum-401k-contribution-limits-for-2018.aspx
A 401(k) must be set up by Dec. 31 of the year for which you make a contribution. This is also true for one-participant 401(k) plans used for self-employed individuals.
The employer has until the business income tax return due date, including extensions, to make the contributions.
for an individual - such as a single member LLC, the due date of the 1040 including extensions.
