Glad to be of service. Let us know how it goes. Oh yeah, post some pictures of your work too!
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!
another thing to consider is if the first peice thats already on the wall is laying at the correct angle on the wall.. meaning is it actually sitting at 45 degrees.. having it off by a degree or two can throw off your mitre cuts angle. check the lay of the crown by nesting a piece of it in a framing square.. when its hitting on the same measurement both on the blade and tongue of the square make a gauge block thats the same dimension..now use it to make small ticks on the wall and ceiling every 16"-24" or so. use the marks as a guideline to keep the crown laying correctly.
another to do is dont nail the last 2' of the first peice of crown.. just let it float.. you can manipulate it and the coped peice until they joint up just perfect.. use thin shims as required to make the joint nice and tight by shimming where the crown hits the wall or ceiling.. dap it to hide any small gaps where it meets the ceiling or wall
fire up the saw and make some dust
Make sure when you're coping, to back cut it just a bit, and leave the piece long. That way you can sneak up on the flat cut on the other side that meets in the opposite corner, or if you can spring it into position the slight back cut will bite a bit into the opposing piece. It also helps you avoid having to make a perfect cut since the back side will never be seen.
i think what was confusing may have been having a compound miter saw to use. While they let you make the cut easier, there is a lot more fiddling to get the angle right. Like others said it's easier to just stick to 45 degrees and make sure the molding is on the fence correctly: base of saw is the "ceiling" and the fence is the "wall."