Scalebar Intervals - Defining Numeric Values

[Matthew Smith]

Hi

I am creating a map of fixed extent and scale (layers can be turned on and
off).  How can I create a scalebar of a fixed length - in km rather than
pixels?  I want to draw a scalebar 100km long; the one created automatically
is 128km long and divided 32,64,96,128km.  This will probably look weird to
those who are either expecing 10s or at least 25,50,75...