26 C.F.R. § 1.642(h)-4
The carryovers and excess deductions to which section 642(h) applies are allocated among the beneficiaries succeeding to the property of an estate or trust (see § 1.642(h)-3 ) proportionately according to the share of each in the burden of the loss or deductions. A person who qualified as a beneficiary succeeding to the property of an estate or trust with respect to one amount and does not qualify with respect to another amount is a beneficiary succeeding to the property of the estate or trust as to the amount with respect to which he qualifies. The application of this section may be illustrated by the following example:
Example. A decedent's will leaves $100,000 to A, and the residue of his estate equally to B and C. His estate is sufficient to pay only $90,000 to A, and nothing to B and C. There is an excess of deductions over gross income for the last taxable year of the estate or trust of $5,000, and a capital loss carryover of $15,000, to both of which section 642(h) applies. A is a beneficiary succeeding to the property of the estate to the extent of $10,000, and since the total of the excess of deductions and the loss carryover is $20,000, A is entitled to the benefit of one half of each item, and the remaining half is divided equally between B and C.
26 C.F.R. §1.642(h)-4