permissible range
(3) Actuarial assumptions must be reasonable; full funding (A) In general Except as provided in subparagraph (B), principles similar to those set forth in paragraphs (3) and (6) of section 431(c) shall apply for purposes of this section. (B) Interest rate for reserve plan (i) In general In the case of a qualified reserve plan, in lieu of taking rates of interest into account under subparagraph (A), the rate of interest for the plan shall be the rate selected by the taxpayer which is within the permissible range. (ii) Rate remains in effect so long as it falls within permissible range Any rate selected by the taxpayer for the plan under this subparagraph shall remain in effect for such plan until the first taxable year for which such rate is no longer within the permissible range. At such time, the taxpayer shall select a new rate of interest which is within the permissible range applicable at such time. (iii) Permissible range For purposes of this subparagraph, the term “permissible range” means a rate of interest which is not more than 20 percent above, and not more than 20 percent below, the average rate of interest for long-term corporate bonds in the appropriate country for the 15-year period ending on the last day before the beginning of the taxable year.