Another idea. Get a sliding T-Bevel. This has a wood or plastic base and a sliding arm that can be moved to any angle (when you hold one, you will see what I mean). Put the wood base up against the wall and move the sliding arm until it lays up against the angle made by the inside or outside miter edge of the piece you will be cutting to match. Then lock the arm in place, go to your power miter box, put the T-bevel base against the fence on the saw and move the blade until the T-bevel arm lays right up against the saw. Now you have the correct angle for cutting your matching baseboard.