I know this is a few years after, but I found this forum in search of cutting crown moulding that had odd inside angles.
After reading what I could find to try to figure out how to do this and getting confused with the answers that were not clear, I realized what I needed to do after trial and error.
You get an Angle Measuring tool, like listed here at this web site,
measure the angle.
Mine was 142 degrees on the first inside angle.
I subtract this from 180 degrees to get my angle I was working with, which I realized I had a 38 degrees angle I had to work with.
I divided this in half and got 19 degrees cutting angle for both pieces to make the 38 degree trim angle.
Holding the crown moulding upside down in my miter saw, I cut the pieces and was able to install nicely.
Each angle afterward, I repeated this formula to get the trim up.
My house walls were not square, so I had angles of 142, 138, 140, & 136 degrees to adjust around.
I would believe it would be wise to check all the inside and outside angles in the house to get good clean cuts.
I was not experienced in this and this was the first time I had attempted this, and was glad that I finally was able to quit beating my head against the wall and figured out how to do this.