This seems to assume that the segment between the pivot point (wrist or elbow) and the butt of the chisel must form a 90° angle with the chisel's shaft. Not being a woodworker – can't one simply move the wrist or elbow slightly higher (thus forming an obtuse angle) to compensate for the depth of the head to ensure that even with a flat mallet head strikes perpendicularly? So mallet head angle becomes purely an ergonomic adjustment to allow better arm placement.