I think you have conundrum of figure what the deduction is if I take the deduction. I think the answer to your question, as posed is - You can't. 

Enter the interest deduction and see what the new interest deduction is.

Use that new interest amount and see what the new interest deduction is..

Use that new interest amount and see what the new interest deduction is.

Use that new interest amount and see what the new interest deduction is.

Use that new interest amount and see what the new interest deduction is.

Repeat until it doesn't change (or something close to that).