What is the best way to calculate the angle at which crown moulding should be cut?
Most crown molding is going to have to be cut flat on the table.  the angles are going to be 52 degrees on the angle, and 38 degrees on the bevel.  For your corners, you are going to want to cope them instead of trying to fit these angles.  Coping is the proper way of installing any type of trim (crown mold, base, shoe molding, cherry rail ect.)

How you would cope your corners is by getting your measurement to the wall.  You will be running the coped end into another piece that is cut square into the wall.  Your wall measurement for your coped piece is going to be the long point of your 52 degree angle, and the long point of your 38 degree bevel.  You will then take a coping saw and cut back into the piece cutting along the short edge of your bevel cut.  Following the pattern of your molding.  A good technique to installing to ensure a tight fit is to:


First make sure your corner fits well into the other piece.
After you have to corner matched nail the molding about 4-5 feet back from the corner to hold the piece up for you.  Also, leave your square cut loose as well.  This way you can manuver both pieces to fit right incase the corner is out of square.
Match your corner and nail where it fits snug.
Nail in the middle of you first nail and the corner. If there is any slack, push the middle in and nail.  This will help seat your corners and give you a nice fluid look.


If you choose to try to angle you corners you are going to spend all day trying to get them right.  It will not look good, and you will waste a lot of time.  With coping you can get away with being a degree or two off without it opening up at the base or top of your corners. 

I have done trim carpentry for 8 years and this is the best way to put up your molding.  It will look great, and you will not have to worry as much about the expansion/contraction of your molding due to season changes.  Also another tip.  On your long runs, make sure you bevel the ends together when joining two pieces in the middle.  Do not ever but up square ends when installing any type of trim.  Happy DIY 