(c) Where, based on the entitlement to benefits of a miner or surviving spouse, a dependent would have qualified for augmentation purposes for a prior month of such miner's or surviving spouse's entitlement had such request been filed in such prior month, such request is effective for such prior month. For any month before the month of filing such request, however, otherwise correct benefits previously certified by the Office may not be changed. Rather the amount of the augmented benefit attributable to the dependent filing such request in the later month is reduced for each month of the retroactive period to the extent that may be necessary. This means that for each month of the retroactive period, the amount payable to the dependent filing the later augmentation request is the difference, if any, between:

1. (1) The total amount of augmented benefits certified for payment for other dependents for that month, and
2. (2) The permissible amount of augmented benefits (the maximum amount for the number of dependents involved) payable for the month for all dependents, including the dependent filing later.