(Last year’s tax bill multiplied by 105%)/365) multiplied by the number of days between last year’s tax bill and closing.

Example: Last year’s tax bill = $1000 and Closing on January 31, 2019.
Tax proration: ($1000*105%)/365= $2.88 (Per diem tax rate).
2018 proration: $1050.
2019 proration (31 days): 2.88 * 31= 89.28.
Total tax proration: $1139.28.

Please note that at various times during the year, one of the tax bill installments will have been paid and the other will not.  Depending on the time of year, this will affect the above formula.