Sure! Glad you're enjoying the game!
Let's take an 8 room dungeon. So when we get to half size (4 waypoints explored), we start rolling a d8 on room 5. If we roll an 8 while exploring waypoints 5, 6, 7, or 8, we will find our goal and can leave the dungeon.
However, let's say we don't find it. So we look at all our unexplored waypoints (places where there were exits we did not take) to search last. At this point, the dungeon can no longer expand. All those waypoints have to either connect to each other, or exist as a dead-end. Let's say we have 3 unexplored waypoints.
- So we move to the first one and roll d8+1 (one room above size) looking for an 8 or 9 (7+1 or 8+1). We don't roll that so we move on.
- In the second one we roll d8+2 (2 rooms above size) looking for an 8 or 9 or 10 (6+2, 7+2, 8+2). We don't roll any of those again so we move on
- The third room doesn't require a roll because it is the last possible waypoint. The dungeon has been fully explored, and so this room must contain our goal. There is nowhere else to explore
This might mean your dungeon goal was right beside the entrance (it has happened to me), but that's a fun "I should've taken the right at Albuquerque" situation that does exist in real dungeon crawls.