Except as otherwise specifically provided in this code, where more than one class of water-reactive solids and liquids is stored in the same control area, the maximum quantity allowed of each water-reactive solid or liquid shall be limited as follows: the maximum quantity shall be reduced by the maximum allowable quantity for each such solid or liquid by the proportion that such solid or liquid bears to the total quantity of water-reactive solid and liquid stored in the control area. The total of the proportional amounts shall not exceed 100 percent.