Section 992. Addition to tax in the case of valuation understatement

In the
  case  of  any  underpayment  of  a  tax imposed by this article which is
  attributable to a valuation understatement, there shall be added to  the
  tax  an amount equal to the applicable percentage of the underpayment so
  attributed.
    (a) For purposes of this section, the applicable percentage  shall  be
  determined under the following table:
 
    If the valuation claimed is
    the following percent of the               The applicable
    correct valuation:                         percentage is:
         fifty percent or more but
         not more than sixty-six
         and two-thirds percent ....................  ten
         forty percent or more but
         less than fifty percent ...................  twenty
         less than forty percent ...................  thirty
    (b)  For purposes of this section, there is a valuation understatement
  if the claimed value of any property is sixty-six and two-thirds percent
  or less of the amount determined  to  be  the  correct  amount  of  such
  valuation.
    (c)  This section shall not apply if the underpayment is less than one
  thousand dollars with respect to the estate of the decedent.
    (d) The commissioner may waive all or any part of the addition to  tax
  provided  by  this  section  on showing by the executor that there was a
  reasonable basis for the claimed valuation and that such claim was  made
  in good faith.