A. Average slope used. When calculating the slope of a lot, an average slope is used based on the elevations at the corners of the lot. The average slope of a lot is calculated by subtracting the average elevation of the uphill lot line and the average elevation of the downhill lot line and dividing the sum by the average distance between the two lot lines. The average elevation of the uphill and downhill lot line is calculated by adding the elevations at the ends of the lot line and dividing by two. See Figure 730-9.