N.H. Rev. Stat. § 21-J:33-a

I. If there is a substantial understatement of tax imposed under RSA 77-A, RSA 77-E, RSA 78-A, RSA 78-C, RSA 82-A, RSA 83-C, or RSA 84-A for any taxable period, there shall be added to the tax an amount equal to 25 percent of the amount of any underpayment attributable to such understatement. II. For the purposes of this section, there is a substantial understatement of tax for any taxable period if the amount of the understatement for the taxable period exceeds the greater of: (a) 10 percent of the tax required to be shown on the return for the taxable period; or (b) $5,000. III. For the purposes of paragraph II, the term "understatement" means the excess of: (a) The amount of the tax required to be shown on the return for the taxable period, over (b) The amount of the tax imposed which is shown on the return. IV. The amount of the understatement as defined in paragraph III shall be reduced by that portion of the understatement which is attributable to: (a) The tax treatment of any item by the taxpayer if there is or was substantial authorization for such treatment; or (b) Any item with respect to which the relevant facts affecting the item's tax treatment are adequately disclosed in the return or in a statement attached to the return.