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,
http://www.sears.com/skill-tech-ins...p-00940760000P?prdNo=1&blockNo=1&blockType=G1 ,
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.