DC's by Level
A very simple tool for assigning DC’s to skill checks based on level is to use the Encounter Design table for determining Challenge Rating (CR).
Refer to Table: Encounter Design to determine the Challenge Rating your group should face, depending on the difficulty of the challenge you want and the group’s APL then use the following formula:
DC = 10 + APL + (APL modifier x 5)
For example, if you wanted a Challenging check for an APL 3 party the DC would be 18.
10 + APL 3 + (Challenging modifier +1 x 5) = DC 18
The DC can be further modified up or down 1-3 points if you want as well.