= "total number of the stitches in one section" / 2 - "1 stitch"
In the diagram above, it requires total 20 stitches to fill one section so I made nine stitches. 20 devided by 2 and minus 1 makes 9.
This is not bi- but quad-coloured scales though all four sections are in the identical colour scheme. In this way you stich four stitching paths in the same colour and switch the colour just for the last stitch. It's certainly more easier than stich the design as bi-coloured scales.