Lot area and width; overall density. For every principal building hereafter erected or used for use as single-family detached dwelling in this district, a lot served by both an approved public sanitary sewer system and an approved public water supply shall have an area of not less than 10,000 square feet and a lot width of not less than 70 feet at the building line and a frontage not less than 25 feet wide at the street line; provided, however, that if a subdivision of a tract is proposed for two or more single-family detached dwelling lots, no more than one dwelling unit shall be permitted for each 20,000 square feet of land of the total overall development tract. In determining the number of dwelling units permitted, the factor resulting from the division of the total gross square footage of the overall development tract by 20,000 shall be rounded up or down to the nearest whole number, with 0.5 or greater being rounded up. (For example, if the total gross square footage of the overall development tract is 230,000 square feet, 12 dwelling units would be permitted (230,000/20,000 = 11.5, rounded up to 12)).