I believed the picture showing the front shield posted on page 25 gave the answer, Michael.
For the upper part of the front extension, giving it a 20 degree angle at the base where it folds, it does indeed come to zero. And to what concerns the top of the lateral partition, yes it's longer than what you thought.
For the latter, I presume you used Eduard's shield as a pattern and we all know by now for being wrong on several places!
Now, if you compare the Squadron pic to your shield, taking into consideration the perspective, you will notice that "A" is shorter than "B"; which doesn't result in your repro. I'd suggest you to take another picture... using the same angle for comparison?
The technique I used here for making the extensions (which is a photographer's trick, back at the time of the good old Hasseblad cameras), was to redraw the shield, the very same one shown on your pic -- as well as an another one on page 27 of the Squadron book -- on a clear transparent sheet of film that I then fixed on my camera's screen. The next step was to put in front of the lens the shield with its wing and to find back the angle the photographer used to shoot his picture.
And... the result spoke for itself. Or at least, I hope it did! Otherwise, we'll have to wait for Robin's release.