42 CFR §417.566

1. (a)Justification. A method of apportionment or allocation of costs, other than the methods prescribed in this subpart may be used if it results in a more accurate and equitable apportionment of allowable costs and is justifiable from an administrative and cost standpoint.
2. (b)Required approval.
   1. (1)An HMO or CMP that desires to use an alternative method must submit a written request for CMS approval at least 90 days before the beginning of the period for which the different method is to be used.
   2. (2)If CMS approves use of a different method, the HMO or CMP may not revert to another method without first obtaining CMS's approval.