Inside corners should almost always be coped not mitered. Any loose fit will be pushed away by the nails opening the joint. If you must miter it the angle is one half the angle of the corner. 90 / 2 = 45. to cut molding put it in the saw upside down and backward. (right end joint cut left end with molding top down).
Just below your sign in block in the upper right corner you will see "search". Click it and type in coping crown molding. There has been endless discussion about this topic here. Just read the replies.