20 CFR § 226.63 - Determining monthly compensation.
(a) Based on yearly compensation. If Board records do not show monthly compensation for a year, the monthly compensation is determined by dividing the total compensation reported for the year by the number of months of service credited to the employee for that year.
(b) For employee with government employment and no railroad service for 60-month period before annuity begins -
(1) General. The compensation used in determining the average monthly compensation (AMC) is indexed for an employee who has not worked in the railroad industry for the 60-month period before the month the employee's annuity begins and whose major employment during that period was for a government agency listed in § 216.16 of this chapter. The compensation is indexed by multiplying it by the quotient obtained by dividing the average annual wage for the indexing year by the average annual wage for the year being indexed. If the month for which compensation is being indexed is before 1951, the average annual wage for 1951 is used.
(2) Indexing year defined. The indexing year is the second year before the year in which the annuity begins.