Quick Hack: Perception Radius

In d20 based games, I like to have NPCs take 10 on their Spot and Listen checks to detect PCs. This gives PCs a static DC to roll against, and if the PC fails he or she is safe until the NPC is within (PC check - DC) x 10 feet.

Spot and Listen skills normally add +1 to check DCs for every 10 feet of distance, and the math above tells you how far away the PC needs to be for the modifier to make up for the failed check.

People tend to forget about this particular modifier which tends to make Spot and Listen unnecessarily good and hurt players—most of the time only monsters that focus on being stealthy try to be sneaky, but the same isn't true for adventurers.

I originally used this in a d20 Modern steam punk espionage game.