Quick update on this -- I can't quite get my head around how to quantify the inherent difficulty of the levels. I was thinking about rating each monster and systematically coming up with scores for each region, but that seems like too much of a subjective and awkward process. Generally, I actually think you've got this pretty much right, although I would up Ice Age to 150%. Sorry I couldn't be of more help, but it's tricky!
Oh, I did have one idea on how to really refine these numbers, though. Put a few polls up trying to find a general consensus on how difficult each area is. Let's say a 10 point scale, and use those numbers to refine the current multipliers. Big problem with this is that it would require numerous polls, though, probably too many.
I'll continue to think about it, but I think you've pretty much got this correct, anyway.