Section 99I. Said commission shall, annually not later than December first, estimate the amount of money required to pay the costs and expenses of the district for the following year and shall fix and determine the proportion of such costs and expenses to be paid by the member towns using the following formula:
Fifty per cent thereof shall be assessed upon all towns in the district in the proportion which the total equalized valuation in each said town bears to the total equalized valuation in the district. When used in this section ''equalized valuation'' shall mean the equalized valuation of the aggregate property in a town subject to local taxation, as most recently reported by the commissioner of revenue to the general court under the provisions of section ten C of chapter fifty-eight.
Twenty-five per cent thereof shall be assessed upon all towns in the district in the proportion which the number of inhabitants in each said town bears to the total number of inhabitants in the district. The number of inhabitants shall be determined in accordance with the last preceding federal census.
Twenty-five per cent thereof shall be assessed upon all towns in the district in the proportion which the total mileage of state highways and town ways in each said town, as most recently determined by the state department of highways, bears to the total mileage of state highways and town ways in the district.