The Town shall pay 50% of the cost of laying out and constructing water pipes or a system or systems of water distribution, after deducting from the total cost any federal or state grants and the remaining 50% of the cost shall be paid by assessment of the cost of such public improvements, to be apportioned among the owners of property fronting on either side of that portion or portions of a public or private way in which such water pipe lays. Such assessment shall be made according to the number of principal structures on such land, and in the case of vacant land, or vacant portions of built upon land, according to the number of buildable lots into which such land may be divided under the Zoning Bylaw. The relative portion of payment provided by this section may be changed from time to time by vote of the Town at Town Meeting.