I am trying to cut crown moulding to fit into a 120 degree angle (corner of two walls). The moulding is a 38 degree type (not 45 degrees). How does one calculate the angles to cut the wood at, using a compound miter saw?
I would place the moulding upside down on the miter saw, so the bottom edge of the moulding is against the fence. Using only the miter angle (no bevel), you can use the same angle as if it were a flat board. Be sure the small flat on the bottom of the moulding is flat against the fence.
If you are coping the joint, you'll use a 60 degree miter, otherwise a 30 degree on both pieces should be close.
It will help to leave the last piece loose for the last 2 or 3 feet so it can be adjusted to match the next piece.