SEP IRA

Have a client single 29 years old, a dentist. Operates through a single member LLC therefore will be filing Schedule C.
In 2018 has gross income of $ 98,000 with business deductions of $ 12,000 excluding self employment tax and potential
contribution to a SEP IRA. Therefore net is $ 86,000 before the SEP IRA and self employment tax expense.

1. What is his maximum SEP IRA contribution? Is it 25% of 86,000 or
2. 25% of $ 72, 842, which is $ 86,000 – 13,158 ( 15.3% {self employment tax} of $ 86,000). Thus 25% of $ 72842= 18,210.50 SEP IRA contribution?
3. Assuming $ 18,210.50 is the right amount does that goes in Schedule C as a business deduction; or in FORM 1040 as an adjustment to get to adjusted gross income
4. Or the answer to determine his SEP IRA contribution and self employment tax is something else?
5. Does the new 20% Section 199A deduction affect this computation