You're thinking in terms of lens length, which will confuse the issue. Instead just think about [distance-to-subject] vs distance between [subject-center] and [subject-edge]. Then it's a simple triangle, and you can use an arc tan to find the angle to the edge, because you know the distance of the opposite side ([subject-center] to [subject-edge]) and adjacent side ([distance to subject]).

For instance if the object is 10cm across (i.e. extends 5cm to the left of the center and 5cm to the right of the center) and you are 5cm away and you choose a lens that fills the frame then the angle to the edge of the object is

45 degrees. If, with the same subject you are 500cm away and fill the frame then the angle will be

0.6 degrees.

To determine what the highest angle you can accept for your project is something I think only you can tell us from practical experiments. But if you determine that angle you can easily replicate it with objects of different sizes by maintaining that same angle.

This is true even if you don't fill the frame. But if you take a wide angle lens and step back 50 meters then the subject will only be a few pixels across

.

If your not a math guy, just click

this link and change the distances to match your scenario.