I use the flatbed scanner in my office a lot and I lay a ruler on there by the Part or flange I’m scanning so I can calibrate it.
I’m wondering if you had a backlit table and then put the part on there like you did for the white paper if it would cancel out the Shadow and give you a little bit more of a correct measurement.
The optical illusion of part of it where you need orthographic projection but you only can get perspective in reality when trying to take a picture of it 1/4 inch part on a piece of paper instead of tracing.
I think if you’re trying to take a picture of a part directly like that having the camera is far away from it as possible and zoomed all the way in will minimize the effects of perspective. In normal operation the trace wouldn’t have to deal with it at all cuz it just cares about 2D.
It’s an interesting thing to think about, the difference in perspective vs orthographic