It's personal taste whether you leave room for a neutral encounter (3-4) or keep it two options (positive: 4-6).
For dungeon reruns, you add 1d6 to calculating the rooms - which yeah 100% it's unclear. The idea is that the 2d6 digits are added together so say you roll a 2 and a 6 - that's 7 rooms. If say it's a rerun and you roll a third d6 which gives you a 4 - on the rerun it's 11 rooms.
The dungeon section is at the top of the list for a rewrite if I ever make a v2. Hope this helps some.