I expected my net income to be reduced by the amount of the contribution, but it didn't. My tax balance due did reduce, though.
No, you shouldn't list your SEP-IRA contribution as a miscellaneous expense. When you enter it under Self-Employed Retirement Plans, TurboTax automatically carries it to Line 28 of your Form 1040.
The SEP-IRA contribution is an adjustment to income, meaning it is subtracted from your total income to arrive at your adjusted gross income (AGI). You can see this for yourself by previewing the Form 1040 and scrolling down to Line 28.
There is a special computation to calculate the maximum deduction if you are self-employed and have your own SEP-IRA. You'll find the details at this IRS link: SEP Plan FAQs - Contributions