Cal. Probate Code § 20114

(a) As used in this section, “qualified real property” means qualified real property as defined in Section 2032A of the Internal Revenue Code (26 U.S.C. Sec. 2032A). (b) If an election is made pursuant to Section 2032A of the Internal Revenue Code (26 U.S.C. Sec. 2032A), the proration shall be based upon the amount of federal estate tax that would be payable but for the election. The amount of the reduction in federal estate tax resulting from an election pursuant to Section 2032A of the Internal Revenue Code (26 U.S.C. Sec. 2032A) shall reduce the tax that is otherwise attributable to the qualified real property that is the subject of the election. If the tax that is otherwise attributable to the qualified real property is reduced to zero pursuant to this subdivision, any excess amount of reduction shall reduce the tax otherwise payable with respect to the other property, this amount to be equitably prorated in accordance with Section 20111. (c) If additional federal estate tax is imposed under subsection (c) of Section 2032A of the Internal Revenue Code (26 U.S.C. Sec. 2032A) by reason of early disposition or cessation of qualified use, the additional tax shall be a charge against the portion of the qualified real property to which the additional tax is attributable, and shall be equitably prorated among the persons interested in that portion of the qualified real property in proportion to their interests.