With a properly set up and sharp block plane you should not need to sand the bottom of the saddle. I've found that the Stanley #60-1/2 P low angle block plane will cut bone reasonably well for a while if the iron is sharpened to a short bevel. Harder irons, such as Hock, tend to chi p on the hard bone. I use my plane to 'shoot' the bottom surface of the saddle. Lay the saddle down on something like a piece of plywood, that is uniform in thickness and holds the bone up off the bench top. A stop on the bench or plywood helps to hold it in place. Have the lower surface of the saddle overhang the plywood. Lay the plane down on the bench take light cuts off the bone. With a well set up plane you can take a nice shaving all the way along, leaving the bottom of the saddle both square and flat. You could use the same technique, of course, using a piece of sandpaper glued to a squared off block of wood instead of the plane, but the plane works better.