A couple of years ago I was doing some trim with my friend Marcos Bradley. He was running base around a series of odd angles -- angles he couldn't readily cut with his chop saw. After some thought he assembled a jig similar to the one shown in the drawing.
Use clamps or screws to secure one of the jig's fences to the saw's fence. Clamp the workpiece to the jig (block under the far end of long pieces) and you're all set to cut accurate acute angles.
M. Felix Marti, Monroe, OR