yields, as published by the Board of Governors of the Federal Reserve System, for the 5-year period ending on September 30 of the calendar year ending before the date with respect to which the estimated value is determined, plus (II) 2 percentage points, for the period beginning on the date with respect to which such amount relates and ending on the date with respect to which the estimated value is determined. (iii) Regulations In the case of any non-readily tradable asset, the estimated value of such asset shall be determined by such method as the Secretary may prescribe in regulations or other guidance. Such method may require a single valuation method with respect to any such asset or may provide one or more options for valuing any such asset and may (but is not required to) include one or more of the following: (I) Required formulaic valuations based on any of the original basis amount (grossed up by a formula), other adjusted cost basis amounts (potentially adjusted by a formula), most recent fair market valuation amount (grossed up by a formula), or formulaic multiple of book value or other financial statement valuation. (II) Any valuation method utilized with respect to illiquid taxpayers under subsection (f), including any method under the special valuation regime and the rule that a valuation may be challenged by the taxpayer only upon a showing of clear and convincing error. (iv) Certain required applications of illiquid taxpayer rules The Secretary may issue regulations or other guidance which require certain taxpayers which hold one or more non-readily tradable assets to apply one or more of the rules applicable to illiquid taxpayers under paragraph (4) and subsection (h) (without regard to whether the taxpayer makes the election described in paragraph (4) or any election under subsection (h)) with respect to all or any portion of such assets. The Secretary may require calculation and payment of estimated annual taxes on such assets to the extent that the Secretary determines that doing so would best