Check every 50 yards (i.e. Speed 1). Roll 1d6. On a 1-2 roll on the chart below.
What Does the Cavern/Tunnel Feature?
What Creature Is Encountered?
How many? Divide the total XP value of the heroes (100 XP per level + 10% XP value of powers) by the XP value of the monster. This number is the max roll on a dice. In other words, if the value is 5, then there are 1d5 monsters (or 1d10 divided by 2). For a really challenging encounter, double the number of monsters rolled on the dice.
And for an example ...