The rate of assessment per square foot in each subdivision of an improvement district shall be fixed on the basis that the special benefits conferred on a square foot of land in subdivisions first, second, third, fourth and fifth, respectively, are related to each other as are the numbers, forty-five, twenty-five, twenty, ten, and five, respectively, and shall be ascertained in the following manner:

(1) The products of the number of square feet in subdivisions first, second, third, fourth, and fifth, respectively, and the numbers forty-five, twenty-five, twenty, ten, and five, respectively, shall be ascertained;

(2) The aggregate sum thereof shall be divided into the total cost and expense of the improvement;

(3) The resultant quotient multiplied by forty-five, twenty-five, twenty, ten, and five, respectively, shall be the respective rate of assessment per square foot for subdivisions first, second, third, fourth and fifth: PROVIDED, That in lieu of the above formula the rate of assessment per square foot in each subdivision of an improvement district may be fixed on the basis that the special benefits conferred on a square foot of land in subdivisions first, second, third, fourth and fifth, respectively, are related to each other as are the numbers 0.015000, 0.008333, 0.006666, 0.003333, and 0.001666, respectively; and the method of determining the assessment on each lot, tract, or parcel of land in the improvement district may be ascertained in the following manner:

(1) The products of the number of square feet in subdivisions first, second, third, fourth and fifth, respectively, for each lot, tract or parcel of land in the improvement district and the numbers 0.015000, 0.008333, 0.006666, 0.003333 and 0.001666, respectively, shall be ascertained. The sum of all such products for each such lot, tract or parcel of land shall be the number of "assessable units of frontage" therein;

(2) The rate for each assessable unit of frontage shall be determined by dividing that portion of the total cost of the improvement representing special benefits by the aggregate sum of all assessable units of frontage;

(3) The assessment for each lot, tract or parcel of land in the improvement district shall be the product of the assessable units of frontage therefor, multiplied by the rate per assessable unit of frontage.