I understand that if a unit's LOS crosses a crest before it crosses a hexside, there is a LOS to a unit at a lower level. What about if the upper level unit is more than one hex back? How far away does a lower level unit have to be to have LOS to an upper level unit at any given point away from a crest?
24.512 Vertical LOS—A unit may only trace a LOS into the
initial Crest hex of each level above it.
This rule seems pretty clear and absolute: I can't see any way a LOS would exist between a lower level unit and an upper level unit which is more than one hex back from the crest line.