Out of necessity, I've developed a pretty sure-fire way to keep the bottom of the saddle flat and square. I double-stick-tape some sandpaper to a slab of granite that's dead flat. If I have a lot of material to remove, I'll just start working the bottom edge of the saddle across the sandpaper, checking periodically to be sure that it's not getting too lopsided or too low. It's all too easy to rock the saddle from side to side a bit unintentionally, which will mess up the flatness of the bottom edge. As I approach my target height, I take the frame from one of those tri-square tools that have a ruler sliding through a frame with one side at 90 degrees to the ruler and the other at 45 degrees. I take the ruler out and hold the saddle flat up against the long side of the the frame, (which is on its side) with the bottom edge of the saddle down on the sandpaper. I'll often put double-stick tape on the outer surface of the saddle (the side not in contact with the frame) so that my fingers can get some traction on it, and continue dragging it back and forth along the frame (on one side) and sandpaper (on the bottom), until I hit my measurement line. This little guide keeps the saddle dead upright, so that the bottom is flat and exactly perpendicular to the sides. Works like a charm. It's slow going, though, so the closer you can get freehand, before using the frame as a guide, the quicker the job will be.
