This question is a little complicated, so I hope I can write it such that it makes sense. The taper at the bottom of the rudder is fairly straightforward: The bottom, horizontal blocking is 3/4" thick at the forward end and tapers to 1/4" at the aft end.
I am having some difficulty understanding the aft, vertical blocking and its relationship to the specially-cut blocking just below the tiller slot. I'm certain the blocking must be taken back to 3/4" thick at the bottom of the tiller slot, but how do I get it that way? I think the most elegant solution would be for there to be a compound taper on the aft blocking so that it gradually becomes 3/4" thick at the top and also tapers slightly from front to back where it is less than 3/4" thick. This would be fairly difficult to accomplish, but I think the best way would be to taper the blocking lengthwise first, then add the width-wise taper as it approaches the bottom. However, if I do this linearly, I will lose the "airfoil" shape before reaching the waterline. So the vertical taper would have to begin at a higher point. I think this is beyond my woodworking skills.
A careful study of the manual, especially the middle picture on page 227, leads me to believe that the aft vertical blocking is the same thickness top to bottom, tapering slightly from fore to aft. It looks as though all the curvature from 1/4" to 3/4" is accomplished on the upper, wider blocking just below the tiller slot. Also, looking at the middle picture on page 225, I see what appear to be pencil lines showing the taper on the bottom edge of the blocking, and an arc showing how the taper will end. It seems to me that this would involve some fairly sharp bending of the plywood, but it sure looks like that is the way it is supposed to be done.
I'm surprised I could find no mention of this in the forum. Maybe no one else had any problem understanding it. Someone please enlighten me!