the mortgage interest limitation is based on the average balance of mortgages for the year. and then there's the added complication when the indebtedness at the beginning of the year predates 12/16/17 and then you sell that home and buy a new one thus having indebtedness post 12/15/2017.
the limit is based on the average balance for the year. so say you had $400K debt for 1/2 the year and $ $800K for the other half. the average for the year is $600K which would make all the interest paid in 2020 deductible.
to further complicate matters the IRS allows computing the average indebtedness balance based on the average for each month. 1/2 the balance at the beginning and ending of each month. then you add the averages for each month and divide by 12
