20 C.F.R. § 226.51
The railroad retirement family maximum is equal to an employee's "final average monthly compensation" (FAMC) up to 1/2 of 1/12 of the annual maximum tier I earnings as shown in part 224 of this chapter in the year the annuity begins plus 80 percent of so much of his or her FAMC as exceeds 1/2 of 1/12 of the tier I maximum in the year the annuity begins. For this purpose, the FAMC is determined by dividing the individual's total earnings up to the tier II earnings limit as shown in part 211 of this chapter for the two highest-earnings years out of the last 10 calendar years, including the year of retirement, by 24. The railroad retirement maximum cannot be more than the FAMC and cannot be less than $1,200.
Example: An employee's annuity begins on December 2, 1982. He has yearly earnings that exceed the tier II annual maximum of $24,300 in 1982 and $22,200 in 1981. The FAMC is the sum of the tier II maximum for 1981 and 1982 divided by 24 ($24,300 + $22,200 ÷ 24) or $1,937.50. The maximum which may be credited to a month for tier I in 1982 is $2,700. The family maximum is $1,350 (1/2 of 1/12 of the annual tier I maximum) plus $470 (80% of the difference between $1,937.50 and $1,350) or $1,820.
20 C.F.R. §226.51