You can't say you had a zero mortgage balance when you didn't have a mortgage. The Average Mortgage Balance is calculated only over the period of time that the mortgage was secured by your home. The mortgage was not secured by your home prior to when the mortgage was originated, so you cannot include the time prior to origination of the mortgage.
For details on how to calculate the home mortgage interest deduction see IRS Publication 936, Home Mortgage Interest Deduction (for 2022). The instructions for calculating the average mortgage balance are on page 12.
