Although I have no experience with Crown Moulding myself, perhaps I can provide some quick help for you (and other experts will no doubt be here soon to answer your question)
You can find a "Crown Moulding Compound Mitre Angle Chart" on Wick's Weekend Warriors Website at http://www.zyworld.com/TheWick/ . Go to the bottom of the page and click on the link.
Also, there has been a lot of discussion about Crown Mouldings right here on the BV bulletin board. You can find posts dealing with this subject by initiating the SEARCH function (Ctrl+F) and entering "Crown" or "Moulding" (or both, for that matter).