Try this for a perfect-fitting apron between tapered legs
After using a tapering jig to make tapered legs for a desk project, I needed to cut the rails that would form the apron between these tapers. At first, I thought I would have to figure out the angle to get the cheek cuts right. Then I realized I didn't need to take chances with guesswork. To make the cheek cuts for the apron's tenons, clamp one of the tapered legs to the crosscut sled and use it as a fence for the apron. After cutting one end of the apron, place the leg on the other side of the blade to make the other cuts. The angle of the leg's taper provides the goofproof angle setting.
—Ray Ladouceur, Cincinnati