Due to nature of the object, accuracy of 0.1mm is non sense. Sincerely there aren't fixed/stable points where you can take so accurate and repeatable measure... this is true even if you would use an ultra accurate calliper!

But you could use a well know reference such as "Regular graphing paper" behind/in contact to your object and take the photo as much as frontal. Finally, count lines between interesting points to get an estimation of distances.

Sure error will be more than 0.1 mm due to the distance between the object and the graphing paper and the angle between the camera and the object.

You could reduce a bit this perspective error using the Camera Calibration Chees Board but, again, expected improvement is non sense

In addiction the camera calibration provides the pix/mm constant but it would require to use the camera and the object from a fixed point.