Premium Rating. The total premium charged to a small group shall be developed using a per-member rating method. For each covered employee and their covered dependents, the premium shall be determined as follows: For each adult age 21 and over, multiply the base rate by the applicable age and geographic area factors; and For each covered child age 0—20, multiply the base rate of the oldest three children by the applicable age and geographic area factors. Composite Billing. Composite billing is available only under the employer choice model set forth in Regulation .06A of this chapter. The SHOP Exchange will determine premiums under composite billing based on instructions issued by the Commissioner.