The penalty for a distance greater than 10 squares is -2.

As Grey_wolf said, concealment does not change the Perception-to-Stealth ratio. As far as creatures are concerned, the only benefit of obscurement or other sources of concealment is that they might allow Stealth rolls to become/remain Hidden. I'd like to add that concealment would justify the DM changing the DC for Perception checks to spot objects or to observe details about a creature. There is no RAW for making that sort of adjustment. Total concealment would even make some specific checks automatically fail -- in complete darkness, you can't see whether a warrior is wearing red or blue unless you have darkvision.