Section 6166(e) provides:

(e) Proration of deficiency to installments

If an election is made under subsection (a) to pay any part of the tax imposed by section 2001 in installments and a deficiency has been assessed, the deficiency shall (subject to the limitation provided by subsection (a)(2)) be prorated to the installments payable under subsection (a). The part of the deficiency so prorated to any installment the date for payment of which has not arrived shall be collected at the same time as, and as a part of, such installment. The part of the deficiency so prorated to any installment the date for payment of which has arrived shall be paid upon notice and demand from the Secretary. This subsection shall not apply if the deficiency is due to negligence, to intentional disregard of rules and regulations, or to fraud with intent to evade tax.

Regulation section 20.6166-1(c) provides:

(c) Treatment of certain deficiencies—(1) No election before assessment of deficiency. Where a deficiency is assessed and no election, including a protective election, has been made under section 6166(a) to pay any tax in installments, the executor may elect under section 6166(h) to pay the portion of the deficiency attributable to the closely held business interest in installments. However, this is true only if the estate qualifies under section 6166 based upon values as finally determined (or agreed to following examination of a return). Such an election is exercised by filing a notice of election with the Internal Revenue Service office where the estate tax return was filed. The notice of election must be filed within 60 days after issuance of notice and demand for payment of the deficiency, and it must contain the same information as is required under paragraph (b) of this section. The notice of election is to be accompanied by payment of the amount of tax and interest, the date for payment of which has arrived as determined under paragraphs (e) and (f) of this section, plus any amount of unpaid tax and interest which is not attributable to the closely held business interest and which is not eligible for further extension (or currently extended) under another section (other than section 6166A).
    (2) Election made with estate tax return. If the executor makes an election under section 6166(a) (other than a protective election) at the time the estate tax return is filed and a deficiency is later assessed, the portion of the deficiency which is attributable to the closely held business interest (but not any accrued interest thereon) will be prorated to the installments payable pursuant to the original section 6166(a) election. Any part of the deficiency prorated to an installment, the date for payment of which has arrived, is due upon notice and demand. Interest for any such period, including the deferral period, is payable upon notice and demand.
    (3) Portion of deficiency attributable to closely held business interest. Only that portion of any deficiency which is attributable to a closely held business interest may be paid in installments under section 6166. The amount of any deficiency which is so attributable is the difference between the amount of tax which the executor has previously elected to pay in installments under section 6166 and the maximum amount of tax which the executor could have elected to pay in installments on the basis of a return which reflects the adjustments that resulted in the deficiency.

Regulation section 20.6166-1(e) provides:

(e) Special rules—(1) Effect of deficiencies and protective elections upon payment. Upon election to extend the time for payment of a deficiency or upon final determination of values following a protective election, the executor must prorate the tax or deficiency attributable to the closely held business interest among all installments. All amounts attributed to installments which would have been due had the election been made at the time the tax was due to be paid under section 6151(a) and all accrued interest must be paid at the time the election is made.
    (2) Determination of date for payment of first installment. The executor may defer payment of tax (but not interest) for any period up to 5 years from the date determined under section 6151(a) for payment of the estate tax. The date chosen for payment of the first installment of tax is not required to be on an annual anniversary of the original due date of the tax; however, it must be the date within any month which corresponds to the day of the month determined under section 6151(a).