You "measured" 49.2125mm? Four decimal places? How? By letting an ant running the distance with a ruler of appropriate ant size?
Seriously, the example shows 15 at the end of the scale. So this seems not to be your example. However, what would it help to have a scale? An app is designed in pixels, you need this for scratch.
Creating a ruler with scale is not what Indesign is intended for. Illustrator should be better for this, or Visio.