"stonej" wrote in message
...
I'm sure it is all over the map depending on cost of ticket,
destination etc. but is there
at least a general rule of thumb say 50% or less full?

It's nothing like that simple. Typically, a short haul plane will start a
A go to B on to C back to B off to D and then back to A (or some such
pattern) If the BD flight is empty but all the others are full it won't
make sense to cancel BD. A long haul flight may simply go A-B-A every day
but even if no passengers have been sold on either leg it may still be
profitable for the airline to operate the plane if sufficient cargo is
available and so long as one leg is sold the round trip will be flown.

Even if the plane simply goes A-B-A short haul it may not get cancelled
unless an alternative is available.