The methodology used under the EQUITABLE SHARING MECHANISM to allocate take-or-pay buyout and buydown costs, to be recovered through fixed charges. Each firm sales customer's fixed charge is determined by comparing its cumulative purchase deficiency with that of the system. The intent is for each customer to bear a portion of take-or-pay responsibility commensurate with its reduced purchases during the period take-or-pay was incurred, as the build-up of take-or-pay liability, and the inability of the pipeline to recover it, resulted from the reduced purchases of its firm sales customers. See EQUITABLE SHARING MECHANISM, DEFICIENCY PERIOD and BASE PERIOD.