Scaling down the modal value when thresholding
Most likely this is chosen by experimental results. Seems like for the algorithm to scceed it needs a value that is lower than the modal value. But not significantly lower (this is where the 1.5 scaling factor comes in). 

I suggest trying the mid-range value and its variants to see how that works. You might be able to skip the scaling procedure.
mid-range = (max value + min value)/2