“Slope” shall be determined according to the following formula:

Where S = average percent of slope; I = contour interval in feet; L = summation of length of all but one contour line in feet; A = area in square feet. The area used shall include the smallest rectangle which totally encloses the footprint of the building, and an extension of that rectangle to an area at least 10 horizontal feet upslope and downslope therefrom. The calculation of the average percent of slope shall be based upon an accurate topographical survey. (Ord. 89-1 § 1, 1989.)