For purposes of paragraph (1)(B)(i), the interest determined under this paragraph for any taxable year is the amount of interest at the underpayment rate under section
6621 plus 1 percentage point on the underpayments that would have occurred had the deferred compensation been includible in gross income for the taxable year in which first deferred or, if later, the first taxable year in which such deferred compensation is not subject to a substantial risk of forfeiture.