In any single year, the license fees paid by City of Portland water and sewer utilities shall not exceed $12,809,321 from the City's sewer utilities, and $4,184,153 from the City water utility until those fee payments equal 5 percent of gross revenues at which point they will increase to equal 5 percent of gross revenues those fees. The directors of the Bureau of Environmental Services and Portland Water Bureau are authorized to adopt administrative rules establishing mechanisms to implement this limitation.