Cutting crown molding with a compound miter saw can be difficult.

The molding will sit flat on the saw table for these cuts. The
angle at which you will set your bevel now depends on the spring
angle of the crown molding you are using. The most common are 45
degrees and 38 degrees.

An easy way to check your spring angle is to take your framing
square and use it as your wall and ceiling and place your molding
in the square.

If the measurement from the corner along the ceiling and wall are
the same then you have a 45 degree spring angle. If the
measurements are different than it is not 45 degree and it is a 38
degree spring angle.

While 45 and 38 are the most common, of those two 38 is the most
common and is most likely what the spring angle is of the molding
you are using.

Sincerely,

Brian Kiernan