Everbody knows that it's very hard to precisely drill through soft materials like dirt/sand/netherrack etc. If you hold mouse button just little too long, it will make complete mess.
So why not add precision mode? By pressing M, you would activate it.
In precision mode, no material will be mined faster than stone. So even if you are mining dirt, you would be mining it with speed of stone.
That way it would be much easier and less tedious to precisely mine through soft materials.