In determining the value of securities, a private foundation may establish that the price at which the securities could be sold outside the usual market may be a more accurate indication of value than market quotations, as a result of-- the size of the block of securities, the fact that the securities are in a closely held corporation, or the fact that the sale of the securities would result in a forced or distress sale. In such a case, any reduction in value for all of these reasons together may not be more than 10 percent of the fair market value of the securities to which the discount applies.