So basically in the first link, they are saying "cut it and fill in the crack with mud". Gee, that doesn't help make a good joint.
Now, I did crown in my theater and I measured so many times, had the right tools including a device that measures inside angles, and I still had to fill the joint.
No matter what, crown is tough to put up right unless you have done a lot of it. So many variations in everything that the crown is up against.
I didn't cope anything, I did all cuts with the mitre saw. There are videos on how to do it on YouTube.
Oh, and if you do use caulk, make sure that it is paintable.
