Under 412(l)(7)©(i)(I), it states "The rate of interest...shall be the rate of interest under (B)(5)..." and then goes on to restrict it within a particular corridor (i.e., if the rate in (B)(5) is outside the corridor, the rate under 412(l) must be at the limit of the corridor).

412(B)(5) includes both the valuation rate for accrued liabilities and the interest rate for the OBRA'87 current liability. Does anyone know of any official guidance that allows you to ignore the valuation rate in the determination of the 412(l) rate? In other words, should you just be looking at the current liability rate under 412(B), or should you be looking at the valuation rate also? I believe the answer is just the current liability rate, but would like to be convinced with official guidance (or even unofficial guidance from an IRS presentation, etc.).

The IRS has given actuaries discretion in choosing the rate within the permissible range. See IRS Notice 90-11. Here is an excerpt:

Until future guidance provides otherwise, for plan years commencing after December 31, 1988, the interest rate that is to be used to determine current liability must be an interest rate that (1) is not less than 90 percent of the weighted average of 30-year Treasury securities (i.e., the lower end of the permissible range) for the relevant period and (2) is not greater than 110 percent of the weighted average of the 30-year Treasury securities (i.e., the upper end of the permissible range) for the relevant period ("qualifying current liability interest rate"). The interest rates specified in the previous sentence are deemed to be consistent with the assumptions that reflect the purchase rates that would be used by insurance companies to satisfy the liabilities under a plan upon plan termination. No interest rate other than those specified in this notice may be used to compute current liability.

Note the range allowed in the law has changed since this notice was issued.

That doesn't answer the question. It only states that whatever result you come up with by applying the law, it is deemed to be the annuity purchase rates referenced under this rule. That still doesn't answer how to apply the law to begin with.