For purposes of subdivision (i) of this subparagraph, the aggregate fair market value of all assets of the foundation shall include the average of the fair market values on a monthly basis of securities for which market quotations are readily available (within the meaning of subparagraph (4)(i)(a) of this paragraph), the average of the foundation's cash balances on a monthly basis (less the cash balances excluded from the computation of the minimum investment return by operation of subparagraph (3)(iv) of this paragraph), and the fair market value of all other assets (except those assets described in subparagraph (2) or (3) of this paragraph) for the period of time during the taxable year for which such assets are held by the foundation. Any determination of the fair market value of an asset required pursuant to the provisions of this subparagraph shall be made in accordance with the rules of subparagraph (4) of this paragraph.
then the price at which the securities could be sold as such outside the usual market, as through an underwriter, may be a more accurate indication of value than market quotations. On the other hand, if the securities to be valued represents a controlling interest, either actual or effective, in a going business, the price at which other lots change hands may have little relation to the true value of the securities. No decrease in the fair market value of any given class of securities determined on the basis of market quotations as provided by subdivision (i)(a) shall be allowed except as authorized by this subdivision, and no such decrease shall in the aggregate exceed 10 percent of the fair market value of such class of securities so determined on the basis of market quotations and without regard to this subdivision.
In computing the income includible under this paragraph as gross income and the deductions allowable under this paragraph from such income, the principles of subtitle A of the Code shall apply except to the extent such principles conflict with section 4942 and the regulations thereunder (without regard to this sentence). Except as otherwise provided in this paragraph, no exclusions or deductions from gross income or credits against tax are allowable under this paragraph. For purposes of subdivision (i) of this subparagraph, the term "gross income" does not include gifts, grants, or contributions received by the private foundation but does include income from a functionally related business (as defined in paragraph (c)(3)(iii) of this section).
26 C.F.R. §53.4942(a)-2