All of this twaddle about perspective is a red herring started by our old conman and jokester Rowbotham. The resolution of the human eye is the only relevant major factor in determining the distance that we can see with the naked eye, unless we take the curvature of the earth into consideration. If we take something the size of a snooker ball and move it to a distance that it is no longer visible to the naked eye, that will be down to the eye's resolving ability and will have absolutely nothing to do with lines of perspective or angles to the vanishing point. It will also make no difference how elevated the eye is in relation to the ball, it will still become invisible at the same distance. However, if we then apply a telescope to the eye, the ball will become visible to the eye once more. This of course is exactly how things should work on a flat earth, if the object is large enough to be within the arc of resolution of the eye, then it should be visible from any distance at ground level. A smaller object would be visible will a suitable magnifying instrument. Just as we can see the light of a star from billions of miles away, we would also be able to see any light source of suitable magnitude, such as the sun or moon, from any point of a flat earth at any time.
The reason we cannot view these things is because the earth is a sphere!
Roger