Parties With a Chance to Surprise
What if a party has a chance to surprise? Let’s walk through the example on page 62 of the DMG. Suppose party A is surprised on a roll of 1. Suppose party B has a 5 in 6 chance to surprise party A. 1 or 2 on 1d6 is the baseline. Since party A is surprised on only a 1, party B’s chance to surprise party A is actually 4 in 6. In other words, party A will be surprised on a result of 1-4 and lose that many segments (unless party B is surprised as well).
A More Complicated Example
What about a gray dwarf—duergar—who surprises others 3 in 6 times, but is only surprised 1 in 10 times (Unearthed Arcana page 10)? Dragon magazine (issue 133, page 74) recommends converting everything to percentages. To determine the number of lost segments, divide the d100 roll by 16 2/3.