I think that the basic mistake you're making is that the bottom of the hill is probably not at sea level (0 ft.). You have to subtract the elevation at the bottom of the hill from the elevation at the top (call this value the "rise").

There's also a slight inaccuracy that you'll get by using the distance along the road from the bottom to the top (.7809 miles) as the value to divide the rise by. Strictly speaking, the grade is "rise over run" where the "run" is the horizontal distance from the bottom to the top. If you remember the terminology of a right-angle triangle, the rise is the length of the vertical part of the triangle, the run is the horizontal bottom leg, and the distance you actually ride along to get up to the top is the hypotenuse. For a typical grade of a few percent, however, the hypotenuse is not much longer than the "run", so it's OK to use that as an approximation.