An overall maximum building height in the zoning district of three stories/35 feet, measured as the average vertical distance from grade to the level of the highest point of the roof surface (exclusive of chimneys and mechanical penthouses which cannot exceed five feet). The reference grade plane for computing height shall be established by the lowest points within the area between the structure and the property line as determined by perpendicular offset from the structure wall, or, where the property line is more than 10 perpendicular feet from the structure, the lowest points between the structure and within 10 feet, or, where a public road exists within 50 feet of the structure, the top of the curb of the public road shall be used. Ground levels are to be averaged at no more than sixty-foot intervals and shall include all building corners.