Cees,

One of the beauties of photography is lenses. You also NEED to know the lens geometry and the position of the lens relative to the subject in order to do what you're trying.

For instance, if I use a lot of light, I get a very large depth of field (area in which my subject is in focus). I get a very sharp image. Unfortunately, that can distort the subject, especially as the subject and camera are closer. You can easily end up with a tapered wing where the tip close to you measures the same (or even WIDER) than the root, while the other wing panel looks like it has extreme taper.

I do close-up photography all the time, and it's something you ALWAYS need to consider when composing a photograph. The photo of the fuse in the box was not well-composed, especially not for what you're trying to do.

You can get decent measurements if your subject is far away and only a small portion of the FOV.

Again, you CANNOT do what you attempted without knowing ALL the geometry: lens diameter, focal length, distance to subject, and angles of lens to subject. I wish you could.

Andy