(a) After a commission order is issued approving the assessment income needed for the watermaster operations for the assessment period under consideration, the executive director shall calculate assessment rates for water use, transport, and storage for each water division, or group of divisions, based on the following formula. Attached Graphic(b) After the assessment rate for municipal use has been determined, the assessment rates for the other uses or for storage must be calculated as the mathematical product of the municipal assessment rate and the rate factor for each use or for storage.