I'm Self-Employed, How Much Of My IRA Contributions Can Be Deducted As A Business Expense?

Typically up to the lesser 25% of net income or $18,000. The worksheet will tell you exactly how much can be deducted.

Find the following worksheet:

Self-Employment Retirement Plan Worksheet

SOURCE:
https://www.irs.gov/publications/p560/ch05.html

I Filed an Extension, Can I Still Make SEP IRA Contributions?

Yes. You have until the end date of the extension to make contributions for the previous year.

SOURCES:
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-seps-contributions