Section 42K. In a city, town or district which accepts the provisions of this section, the water commissioners may assess betterments in accordance with the provisions of chapter eighty for the construction and connection of water mains and services by a uniform unit method which shall be based upon the common main construction costs divided among the total number of existing and potential water units to be served after having allocated the town contribution, if any, and after having proportioned the cost of special (specific unit) and general benefit facilities. Each water unit shall be equal to a single family residence. Potential water units shall be calculated on the basis of zoning in effect at the date of assessment. Existing and potentially and potentially multi-family, commercial, industrial and semi-public uses shall be converted into water units on the basis of residential equivalents.