26 CFR 26.2642-5 – Finality of inclusion ratio
(a) Direct skips. The inclusion ratio applicable to a direct skip becomes final when no additional GST tax (including additional GST tax payable as a result of a cessation, etc. of qualified use under section 2032A(c)) may be assessed with respect to the direct skip.
(b) Other GSTs. With respect to taxable distributions and taxable terminations, the inclusion ratio for a trust becomes final, on the later of—
(1) The expiration of the period for assessment with respect to the first GST tax return filed using that inclusion ratio (unless the trust is subject to an election under section 2032A in which case the applicable date under this subsection is the expiration of the period of assessment of any additional GST tax due as a result of a cessation, etc. of qualified use under section 2032A); or
(2) The expiration of the period for assessment of Federal estate tax with respect to the estate of the transferor. For purposes of this paragraph (b)(2), if an estate tax return is not required to be filed, the period for assessment is determined as if a return were required to be filed and as if the return were timely filed within the period prescribed by section 6075(a).