20 CFR §404.333

Your wife's or husband's monthly benefit is equal to one-half the insured person's primary insurance amount. If you are entitled as a divorced wife or as a divorced husband before the insured person becomes entitled, we will compute the primary insurance amount as if he or she became entitled to old-age benefits in the first month you are entitled as a divorced wife or as a divorced husband. The amount of your monthly benefit may change as explained in § 404.304.