(1)The applicable rate by reference to which a basic amount of estate income is grossed up for the purposes of sections 656 and 657 depends on the rate at which income tax is borne for the tax year by the aggregate income of the estate.
(2)If the aggregate income of the estate all bears income tax at the same rate, the applicable rate is that rate.
(3)If—
(a)different parts of the aggregate income of the estate bear income tax at different rates, and
(b)the same rate applies to all the income from which section 679 treats the basic amount as having been paid,
the applicable rate is that rate.
(4)If—
(a)different parts of the aggregate income of the estate bear income tax at different rates, and
(b)different rates apply to different parts of the income from which section 679 treats the basic amount as having been paid,
each of those rates is the applicable rate by reference to which the corresponding part of the basic amount is grossed up.