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.

1. (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
2. (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.
3. (c) This section shall not apply if the underpayment is less than one thousand dollars with respect to the estate of the decedent.
4. (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.