20 CFR § 404.373 - Parent's benefit amounts.

Your parent's monthly benefit before any reduction that may be made as explained in § 404.304, is figured in one of the following ways:

(a) One parent entitled. Your parent's monthly benefit is equal to 82 1/2 percent of the insured person's primary insurance amount if you are the only parent entitled to benefits on his or her earnings record.

(b) More than one parent entitled. Your parent's monthly benefit is equal to 75 percent of the insured person's primary insurance amount if there is another parent entitled to benefits on his or her earnings record.