The table of the saw is the ceiling and build a jig that will hold the molding on the correct angle against the fence. Just build a box like the old handsaw jig, but to fit the molding. Bosch makes a neet angle finder. Drywalled corners are never square.
Once you have that you just have to remmember that everything is upside down and backwards.
If you go with a molding to big to stand up like that you have to set both angles on the saw and that gets really tough.