Well a very simple answer would be you have to pay as much interest as your total income to reduce your income to zero . But that will not reduce any self employment tax if you have a net profit on schedule C.
If you have the Desktop program you can do another test return, go up to File-New or save your return with another name to test in, go to File-Save As.
In Desktop there is also a What-If worksheet.
Go to Forms Mode, click Forms in the upper right or on the left for Mac. Then click Open Forms box in the top of the column on the left. Open the US listing of forms and towards the bottom find the What-if worksheet. It's right under Estimated Taxes.
