I had an idea last week that perhaps the best way to approach challenge dungeon gearsets would be to stay as close to ilvl 463 as possible. My thinking was based on the way hit/expertise don't scale down, so your rating mix outside of those two will trend towards an even mix. Since some rating is better than another one this has to be bad. But is this actually true? I decided to look up some numbers and see...
The first thing I noticed is even at the same ilvl a piece of gear can have a different rating ratio. The bigger the cap between the two ratings, the smaller the total. I'm sure there's some precise formula but that's something to look up later on. It looks like the biggest split on ilvl 463 gear is 60-40 so I'm going to make some estimates using that split even though it isn't really available in every slot. There's also complications with sockets that I'm going to ignore for now.
All told, assuming primary stat trinkets, you'll have 11116 rating across all your gear with ilvl 463 gear. You need 2550 expertise rating and 2550 hit rating to hit the respective caps. This leaves 6016 rating left, which is actually less than 60% of 11116. So, assuming the gear existed, you could quite feasibly only run with hit, expertise, and your best rating.
Now, assume you have really high ilvl gear. Really, really high level. So you actually have a rating pool of 20000. You still need the same 5100 rating for hit/expertise which leaves 14900 rating to go into other stats. Because the most you can get on any single piece is 60% you're looking at getting at most 12000 of it, leaving 2900 to go into a different rating. Then your gear gets scaled down to having 11116 rating. This means you get only 4845 of your best rating while getting stuck with 1171 of your second best rating. So having significantly higher ilvl gear actually makes you worse!