Mortgage interest limitation when purchasing another primary home mid-year

[ptax255]

Hello, I have a client that had his primary residence with a mortgage $800K post 2017. In December 22, they closed on another home to move in with a mortgage $1.1M. What's the best way to calculate the average balance in this case? When I enter the 2 loans in the mortgage interest worksheet, it calculates an average of balance of $1.8M which seems too high considering the second home closed at the end of the year. Thank you!

[sjrcpa]

Calculate on a monthly basis.

[ptax255]

Thanks. Since the client closed the larger mortgage on 12/22, can I prorate this month's average balance. Or do I take the balance at the beginning/end of the month?