I plugged your statistics into Excel. The halfway change isn't at all visible:
I also played around with a solution, and came up with such a chart:
The sum adds up similarily, and the idea was that Easy is linear, Medium is not much more than Easy (remember that easy has additional benefits!) and UV needs to have a high capacity quick. Things of note:
-- UV must have lots of enemies in the beginning -- former foo's are not that scary with a shottie, and things start to get messy once you come to the stronger enemies. Let it feel like UV :)
-- UV should be more based on the apperance of stronger monsters, not on the sum of their levels.
-- Easy is easier than Medium anyway, because of the rule changes
-- UV's difficulty over Hard is mainly in the earlier appearance of stronger characters -- to make the levels at least a bit populated with the biggies in it it needs a fairly high DL level all round
And another question -- how should the types affect the DL? We have the basic current types:
-- Normal
-- Cave (fairly open area, one type of enemies however)
-- Maze (very cluttered area)
-- City (more open area than standard? Or similar?)
-- Arena (very open area)
It seems logical to decrease Arena DLevel sum, because you have them all going at once. But how much? And what about the others? What about the amounts? Maybe make some calculations for caves?