If your SS benefit is reduced due to compensation earnings in excess of the maximum allowed when you’ve begun benefits prior to FRA, the total of all reductions will be used at your FRA to adjust up your monthly SS benefit such that the total of past reductions will be repaid to you over your life expectancy. How this is calculated I do not know, but what is important to remember is benefit reductions prior to FRA will be gradually returned to the beneficiary when they reach their FRA