A simple method of getting the correct angle on plywood for scarfing is to stack the plywood up with the overlap being the amount of cut to take off. For example if using 1/2" ply and desiring a 8:1 scarf, the set back would be 4 inches (1/2 * 8 = 4). Then use plane, belt sander or whatever tool to trim off the edge of the plywood. The glue lines in the plywood will provide a great indication of the accuracy of your cut. When the lines are parallel and equidistant then you have a flat cut. Of course the lowest part of the original edge of each piece should not be removed.
