Once you know the MFD (minimum focus distance) for a particular lens, is that the distance from the subject to the front of the lens or to the camera sensor?
The Minimum Focus Distance is measured from the image plane that is occupied by either the film or sensor. In fact, all focus distances are measured from this point. Although not defined as such in the academic field of optics, in the field of photography this plane is also referred to as the focal plane.
Many cameras, including most film SLRs and DSLRs, have an external marking that indicates where that plane is using a symbol that resembles the Greek letter Φ. The distance from the front of the lens to the subject is usually referred to as the working distance. When doing Macro work, it is not uncommon for the MFD to be only slightly longer than the physical length of the lens plus the camera's registration distance (the distance from the image plane to the lens flange), leaving very little space between the front of the lens and the plane of focus.