The grades given in this chapter shall be the grade of the top of the sidewalks at their inner edges when the latter are on the block lines or not more than two feet (2') out therefrom. When the inner edge of the sidewalk is more than two feet (2') out from the block lines, the elevation of top of the inner edge of walk shall be determined by subtracting from the established grade at the block line, an amount equal to the amount of fall from the block line to the inner edge of sidewalk, which shall be at the rate of one-fourth inch (1/4") per foot from the block line to the outer edge of curb. (Prior code §24-16)