4.0 Dungeons are quite easy to have 1 LB per boss most of the time at level 70 now.

When I play SAM during the level 70 dungeons I can Lb1 during 1st or/and 2nd boss for each level 70 dungeon and always still build up a LB2 at the Final Dungeon boss once they hit their 10% HP mark then use the LB2.