Most mitre saws have a limit of 45 degrees to which they can be adjusted. This can pose a problem should you need to create a larger angle. With a little math and some basic carpentry knowledge, you can easily solve the problem. This process will work for all manner of angles beyond 45 degrees, however you will have to do the math.
Adjust the mitre saw to 20 degrees and cut the end of a small piece of 1x4 lumber.
Use a table saw (or the mitre saw if you can secure the 1x4 piece safely) to cut the1x4 straight through, opposite the 20-degree cut. This will leave you with a triangular piece of wood that is a right triangle with a 20-degree angle.
Adjust the mitre saw to 45 degrees.
Place the 20-degree block of wood so that the small point is in toward the blade. If possible, clamp this piece in place.
When you place a piece of wood against this block, the resulting angle of the wood to the blade will be 65 degrees.
The bigger you can make the 20-degree block, the better. Small pieces of wood can easily jump off a saw blade and injure you.
Always take extreme precautions when working with power saws. Wear eye protection, and use push sticks and clamps whenever possible to keep your hands as far away from the blade as you can.