Value of interest in a closely held business which is included in the gross estate (C) ÷ Value of gross estate (D) * Gross Federal estate tax reduced by the credits authorized by sections 2011 through 2014 and any death tax convention (B).
The executor may elect to pay in installments an amount less than the amount computed under the limitation in this paragraph. For example, if the total estate tax payable is $100,000 and the amount computed under the limitation in this paragraph is $60,000, the executor may elect to pay in installments some lesser sum such as $30,000, in which event the executor must pay $73,000 to the district director on or before the date prescribed by section 6151(a) for payment of the tax. Of such payment, $70,000 represents tax which the executor either could not elect to pay in installments or did not choose to so elect, and $3,000 represents a payment of the first installment of the tax which the executor elected to pay in installments.
26 C.F.R. §20.6166A-1