Rather than finding the distance with code, I feel it would be easier to use imtool and use the 'Measure distance' tool.
You can run imtool by running the snippet below
imtool('image.png')
This will give you the relative distance in pixels. You can convert this value to mm by using the 'Measure distance' tool on the scale you have here, and calculating the ratios.
Refer to this answer for more details
