I have an object that is 0.6 meter in diameter that stand at 1000 meter away from me, I have a scope with magnification of X15-40.
How can I calculate the size of the image that I will see through the scope at the minimum and maximum magnification values?
The angular size of the image will be: $$ 2\arctan\left(X\cdot\frac{0.3}{1000}\right)\approx\frac{0.6X}{1000}. $$
In other words the object will seem to be $X$ times closer.