Is there a formula? Please help
If you are laying the pieces flat on the saw bed, you need a crown molding degree chart. These can be found on line. There is no One-fits-all formula because not all crown molding is made to fit the wall and ceiling at the same angle. If at all possible learn to cope one inside corner piece to the other one that has been cut square and fitted tight into the corner. The cope is done by cutting a miter cut, as if you were mitering both pieces. For this cut, it is easiest to flip the crown so the ceiling side touches the saw base and the wall side touches the back fence. This is where the crown stops mentioned in previous replies comes into play. I usually draw a line on my saw base instead so I can rock the piece a little for fine tuning. Once you have this miter cut, the fun begins. You are going to cut away the end grain wood inside this miter cut line. How precise you are in cutting exactly on that miter line determines how well the piece will mate to the other piece. Most people use the tried and true coping saw for cutting along this line, then using files, dremmel tools, grinders or other inventions to get the exact fit. If you are working with unfinished wood it helps to take the edge of a pencil lead and slide it along this miter line to give a dark profile to cut along. You will be measuring for this piece wall to wall. You will cut your miter and, before cutting out the cope, hook your tape to the shape edge of the miter, the wall side, and pull to the other end and mark the length. If the pieces are long enough to be flexed, I usually add about 1/16th -1/8". This allows for a force fit and a little trimming. Trimming should not be necessary, though. If anything, tap the square corner piece at the top or the bottom to rock it a little to match your cope. I could write pages about all the little tricks to do a good, efficiant crown molding job. This is a start.