How many ways are there of filling the bin.  The ways involving non-bin
   parts of the histogram are a constant factor.
   The bin is filled by a selection from the sampling events.  Any five of
   the sampcount events may be involved in filling the bin.  The sampcount
   events happen always in the same order, so I don't think we want
   permutations of them ...  I'm guessing that this is a combinations
   situation.  Where using the concept of combinations will give the
   proportional number of samplings that fill the bin to that height.