If a trustee determines that an allocation between principal and income required by section 18, 19, 20, 21 or 24 is insubstantial, the trustee may allocate the entire amount to principal unless 1 of the circumstances described in subsection (c) of section 4 applies to the allocation. This power may be exercised by a cotrustee in the circumstances described in subsection (d) of said section 4 and may be released for the reasons and in the manner described in subsection (e) of said section 4. An allocation is presumed to be insubstantial if:
(1) the amount of the allocation would increase or decrease net income in an accounting period, as determined before the allocation, by less than 10 per cent; or
(2) the value of the asset producing the receipt for which the allocation would be made is less than 10 per cent of the total value of the trust's assets at the beginning of the accounting period.