However, if all you want to find is the ANGLE on your steps, drink a beer and buy a compass.
Make 2 marks on the edge of your deck. One on each side of the corner. The marks should be at 1 foot away from the corner, and be as precise as possible. Now, from each mark, draw a very light circle whose diameter will intersect with the coresponding circle from the opposite mark. Draw another circle slightly larger, so you have two intersections. The line between these intersections are the bisection of the corner, and that is your angle.