20 C.F.R. § 418.2322
If we make an income-related monthly adjustment amount determination for you for the effective year under subpart B of this part (Medicare Part B Income-Related Monthly Adjustment Amount), we will apply that income-related monthly adjustment amount determination under this subpart to determine your Part D income-related monthly adjustment amount for the same effective year. Therefore, if you obtain Medicare prescription drug coverage in the effective year after we make an income-related monthly adjustment amount determination about your Medicare Part B, the income-related monthly adjustment amount determination we made for your Medicare Part B will also apply to your Medicare prescription drug coverage. Any change in your net benefit due will be accompanied by a letter explaining the change in your net benefit and your right to appeal the change.
20 C.F.R. § 418.2322