IRS form 936: What is average loan balance for December loan?

I need help with IRS form 936.  I took out a new mortgage in December 2018 that exceeds $750,000.  I paid interest at the end of December and TurboTax tells me I need to fill out form 936 by hand to get the Allowable/Adjusted Interest I can deduct.

How do I calculate average balance (for lines 7 and 12 of 936)?  It doesn't seem I can use the first & last balance method, since I had no balance in January 2018.  So do I use the 'Interest paid divided by interest rate method'?  That also doesn't seem right because I only paid interest for one month.

Thank you very much.