Miter to picture-frame-perfect length the first time
I've had great success cutting perfect miters for picture frames using a dedicated sled with a plastic 45° drafting triangle acting as a fence, as shown. After squaring the sled base to the guide bar, I cut a saw kerf two-thirds of the way through the sled. Next, I centered a fence a little shorter than the longest side of the triangle on the back edge of the sled. Finally, I screwed the triangle to the sled so that the point of its 90° angle splits the saw kerf.
To use the sled, I first crosscut all my frame sides to finished length--plus 1/8" to account for the kerf of my tablesaw blade. Then, I cut a scrap of stock the same width as the frame workpiece and hold a scrapwood stop against it as shown. With the workpiece butted against the stop, the blade removes half of the 1/8" excess length with each cut. It's as easy to cut on the right side of the blade as it is the left with no adjustments like you would have on a mitersaw.
-- Niki Avrahami, Garwolin, Poland