That'll work much better than just having the piece against the fence for sure. And yes you do need to have the piece upside down, makes it so much easier to see where your mitres need to end up

This is how I did it when I installed kitchens. It can be tough to hold the crown steady again the fence so you can get an attachment for the saw called a crown stop that holds the top edge(which would be against the base of the saw-upside down) steadily to keep the cuts consistent