Pre-Installation Tolerances. A meter may not be placed in service if it registers less than 90 percent of the water passed through it at the minimum test flow or over-registers or under- registers more than 1-1/2 percent at the other test flows. Calculation of Error. The percentage by which the correct registration varies from the meter registration is derived by stopping the meter test hand at the starting point and then determining the percentage variation in registration as indicated by the working standard. The formula for determining the percentage error in registration is: Error in Registration for Billing Adjustments. The error in registration of a meter for billing adjustments shall be the algebraic average of the error in registration at the following two rates of flow: Approximately 10 percent of maximum normal flow; Approximately 50 percent of maximum normal flow, or at the maximum flow available at normal system pressure. Application of Error for Billing Adjustments. The meter error derived in §C, of this regulation, shall be used to calculate the correct consumption of water by applying the derived error in registration to the volume of water consumption indicated by the meter. If the meter is fast then the volume of water indicated by the meter registration shall be reduced by applying the calculated error in registration. If the meter is slow then the volume of water indicated by the meter registration shall be increased by applying the calculated error in registration.