When the flash is not attached to the camera, there is no reason to assume that the optimal flash coverage pattern has anything to do with the focal length of the lens, because the flash could be placed in front of the camera (meaning you need to widen the coverage) or behind it (requiring a narrower flash pattern). That's why Canon didn't bother with auto focal length/flash coverage sync when the flash is off-camera. Set the flash pattern manually to whatever is appropriate for the lighting setup you're using.
