Position the cove molding to be cut. Turn the cove molding upside-down and hold it securely against the vertical fence of the table saw.
Secure its position on the table saw with a C-clamp.
Set the saw at a 45-degree angle. Make the first cut. The bottom edge of the molding will be the longest part so it will fit into the wall corner.
Check to see that the bottom edge of the molding is the longest part of the molding. This way the bottom edge will fit into the corner and the shorter top edge will abut the surface of the ceiling.
Sand the cut edge with the sandpaper to create a smooth surface.
Position the angle finder against the angled area where you'll be installing the cove molding. Note the degree of the angle.
Subtract the number of your reading from 180. Divide this number by 2. This is your cut angle.
Set the table saw for the number you calculated, and cut the cove molding at this angle.