How do you calculate an Epic monster's Challenge Rating? I've been looking through the Epic Level Handbook and I've found nothing. Little help, please?
I have only run epic games twice, so I am not an expert on the epic rules by any stretch of the imagination, but I have found that CRs are not nearly as useful at epic levels as they are at say levels 3 through 15. I think at Epic levels, you and the party should be so experienced with each other that CRs are not needed as much any more. CRs are really good for 2 things IMO, for awarding XP and for coming up with an appropriate level of challenge that is fun for the party, but not impossible.
At epic levels, I tried to cater to what I had in the party. I made encounters that were tough for them, based on what I had seen them do previously. This generally meant that I started out "easy" and let them build their confidence as I got to understand how powerful they were and what their weaknesses were. For me, it was more interesting to just build encounters without concern for CR, since by then I knew what the party could and could not handle.
A 28th level party might be able to take on and kill a CR 40 monster out in the open where they can fly and it cannot, but not be able to defeat a CR 30 monster in another situation where their tactics are more limited by the environment it is in. If the CR 40 is an "easy" victory, I am not going to award them 200,000 xp or whatever the book says for 12 levels higher, just because they caught it out in the open and not in its lair.
As a DM, I try to award xp so that the party progresses at about 4 sessions a level if they are truly brilliant and about 6-7 sessions a level if they are average. At 28th level, this means somewhere between 4 and 7k xp a session based on how they play, not what the book says they should get for CR such and such.
I am considering running another epic campaign, and if someone else has had better success using the epic CRs, it would be great if they chimed in. I try to follow the books as closely as I can, but this is just one area that I had too much difficulty with, but then I have a lot more experience running the many hundreds of monsters at regular levels than I do at running the dozens of monsters at epic levels.