citizenship (within the meaning of section 877) or with respect to whom an event described in section 877(e)(1) occurs, and such heir does not comply with the requirements of subsection (g), or (D) the principal place of business of a trade or business of the qualified family-owned business interest ceases to be located in the United States. (2) Additional estate tax (A) In general The amount of the additional estate tax imposed by paragraph (1) shall be equal to— (i) the applicable percentage of the adjusted tax difference attributable to the qualified family-owned business interest, plus (ii) interest on the amount determined under clause (i) at the underpayment rate established under section 6621 for the period beginning on the date the estate tax liability was due under this chapter and ending on the date such additional estate tax is due. (B) Applicable percentage For purposes of this paragraph, the applicable percentage shall be determined under the following table: If the event described in  paragraph (1) occurs in the  following year of material  participation: The applicable percentage is: 1 through 6 100 7 80 8 60 9 40 10 20. (C) Adjusted tax difference For purposes of subparagraph (A)— (i) In general The adjusted tax difference attributable to a qualified family-owned business interest is the amount which bears the same ratio to the adjusted tax difference with respect to the estate (determined under clause (ii)) as the value of such interest bears to the value of all qualified family-owned business interests described in subsection (b)(2). (ii) Adjusted tax difference with respect to the estate For purposes of clause (i), the term adjusted tax difference with respect to the estate means the excess of what would have been the estate tax liability but for the election under this section over the estate tax liability. For purposes of this clause, the term estate tax liability means the tax imposed by section 2001 reduced by the credits allowable against such tax. (3) Use in trade or business by family