In applying the child support guidelines for an obligor who is receiving social security old age benefits and who is required to pay support for a child who receives benefits as a result of the obligor’s receipt of social security old age benefits, the court shall apply the guidelines by determining the amount of child support that would be ordered under the child support guidelines and subtracting from that total the amount of benefits or the value of the benefits paid to or for the child as a result of the obligor’s receipt of social security old age benefits.