Rockler tool has a Compound Miter Jig that eliminates all angle guessing for cutting crown molding. It sits right on the miter saw and you only have to set miter angles. No need for charts or bevel cuts. It's simple, fast and accurate.
If you have angles such as 120 degrees, you just subtract 90 and divide by 2. This gives you the miter angle of 15 degrees. You set the saw and put the molding in the jig and you're all set. It's really that easy.