Why would you assume the math needs to be different for skill challenges as opposed to single checks?
With bounded accuracy the paradigm has changed from 4e where DCs were by level to set numbers. The advice is to use the difficulties in the PHB, pg 174.
Skill Challenges involve resolution after multiple checks.
A 5th level character with +7 mods against DC 15 medium = 65% of success.
A 5th level character trying to get 8 successes before 3 failures at DC 15 = 26% chance of success.