2018 Numbers
Net Business Profit 61,762
Deductible Self-employment tax 4,363
Adjusted Net Business Profit 57,399
Maximum Profit Sharing 11,480
Employee Compensation 45,919
Employee Salary Deferral - 18,500
Salary Deferral to 403b at day job ** + 2,037
Employer Profit Sharing - 11,480
Employee After Tax Voluntary Contributions 17,976
Since I'm over 50, I'm allowed a $6,000 catch up to the Salary Deferral, so I contributed to the Solo 401k an additional $3,963, totaling $24,500 of salary deferral across all jobs.
The $6,000 catch-up contribution is not included in the annual addition limit. This applies to both the 415c 2018 $55K statutory limit and the 100% of compensation limit.
The annual addition limit specifies that the sum of contributions can't exceed the employee compensation.
The 415c statutory limit specifies a set number that can't be exceeded for all contributions. For 2018 this is 55,000 not including the catch up provision.
For 2019 this is 56,000.
** If instead the $2,037 salary deferral to the 403b at my day job was a $2,037 employer contribution to a 403b plan, then my after tax voluntary contributions would have to be lessened by $2,037. Also, making sure to take some salary deferral at my W-2 job gives me more space for after tax voluntary contributions.