Brad from Surry, B.C. has strange angles to his walls, and he wants to put crown molding around the top. How to cut the angles?
Don't even try with your compound angle power miter box. Build a simple jig. A piece of plywood for the base, which represents the ceiling. (The molding is always cut up-side-down, using the base of the jig as the ceiling). Screw a couple of 2x4 or 2x6's to the base to match the angle of your wall corner, standard or odd angle, no matter. Screw 1x1 stops to the plywood placed so that the molding sits in the jig at the proper slope. Then take a hand saw and cut carefully down the middle of the angled fence. This is the only careful operation -- making sure to cut the angle in half.
Now put one piece of molding in place and rough cut its angle, using the slot between the two fences as a miter box. Move it out and do the other. Then put them both into place, with clamps or a helper's hands, and make a final saw cut down the joint, cutting a little off of each side to "marry" the two pieces. They could be less than the perfect angle, but they mate. If you must fit perfectly corner to corner, do all of this just a bit too long, check for fit and then cut them a little shorter. It is easier to shorten boards when you have worked out the angle, than try to lengthen them.
For a more complete overview including a mitre saw mitre/bevel angle chart, check out Cutting Crown Molding With A Mitre Saw.