Section 409A(1)(B)(ii) says:
(ii)Interest
For purposes of clause (i), the interest determined under this clause for any taxable year is the amount of interest at the underpayment rate 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.
Quarterly interest rates are listed here:
https://www.irs.gov/payments/quarterly-interest-rates
Via Google search I found this article discussing the calculation:
https://www.aon.com/attachments/human-capital-consulting/Premium_Interest_Tax_for_409A_Failures_JDC_...
