I would like to put this out for argument that there is no such thing as magnetic flux density. Since magnetism is a wave like on water's surface, it is continuous. Is there a 'water wave density?" I think not. Unless you mean the distance between wave peaks. But flux lines are only arbitrary man made conventions with no basis in reality so you can't say the this flux line is closer to the previous than the subsequent one. Flux lines are continuous. So where does the 'density' come in to play?