The way blade width is measured will influence your final results.Spine width is 5.16mm and blade width is 15.24mm. If you toss those into a calculator you’ll get around 19.5*.
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Are you measuring the width from the tip of the edge to the back of the hone wear?
Or, is the width you're using going from apex all the way to the very back of the spine?
For what its worth, even if that 19.5 degrees angle is accurate that shaving angle is within a range that could still function well enough.