Cal. RTC § 75.41

(a) The auditor shall apply the current year’s tax rate, as defined in Section 75.4, to the supplemental assessment or assessments, computing the amount of taxes that would be due for a full year. If the tax rate for the “roll being prepared” is known, the rate may be used with respect to the fiscal year to which it applies, rather than the current year’s tax rate as defined in Section 75.4. If the tax rate for the “roll being prepared” is not known, the current year’s tax rate as defined in Section 75.4 shall be used. For property on the supplemental roll, the taxes due shall be computed in two equal installments.

(b) The taxes due shall be adjusted by a proration factor to reflect the portion of the tax year remaining as determined by the date on which the change in ownership occurred or the new construction was completed. In computing the portion of the tax year remaining, the change in ownership or completion of new construction shall be presumed to have occurred on the first day of the month following the date on which change in ownership or completion of new construction occurred.

(c) (1) If the presumed date specified in subdivision (b) is February 1, the taxes on the supplemental assessment for the current roll shall be multiplied by 0.42, and the taxes on the supplemental assessment for the roll being prepared shall be multiplied by 1.00.

(2) If the presumed date specified in subdivision (b) is March 1, the taxes on the supplemental assessment for the current roll shall be multiplied by 0.33, and the taxes on the supplemental assessment for the roll being prepared shall be multiplied by 1.00.

(3) If the presumed date specified in subdivision (b) is April 1, the taxes on the supplemental assessment for the current roll shall be multiplied by 0.25, and the taxes on the supplemental assessment for the roll being prepared shall be multiplied by 1.00.

(4) If the presumed date specified in subdivision (b) is May 1, the taxes on the supplemental assessment for the current roll shall be multiplied by 0.17, and the taxes on the supplemental assessment for the roll being prepared shall be multiplied by 1.00.

(5) If the presumed date specified in subdivision (b) is June 1, the taxes on the supplemental assessment for the current roll shall be multiplied by 0.08, and the taxes on the supplemental assessment for the roll being prepared shall be multiplied by 1.00.

(6) If the presumed date specified in subdivision (b) is July 1, no supplemental assessment shall be made on the current roll, and the taxes on the supplemental assessment for the roll being prepared shall be multiplied by 1.00.

(7) If the presumed date specified in subdivision (b) is on or after August 1, and on or before January 1, the following table of factors shall be used:

The taxes on the supplemental
If the presumed assessment on the current
date is: roll shall be multiplied by:
August 1 0.92
September 1 0.83
October 1 0.75
November 1 0.67
December 1 0.58
January 1 0.50

(d) After computing the supplemental taxes due, if the total is twenty dollars ($20) or less, the auditor may cancel the amount as provided by Section 4986.8.

(e) If the supplemental assessment is a negative amount, the auditor shall follow the procedures of this section to determine the amount of refund to which the assessee may be entitled.