Question

1. So, -1/2 would be marked at a point
half the distance between 0 and -1.​

Answer 1

From the two green lines on the photo, as well as the camera's field of view, you can determine the visual angle α0α0 of the slope as seen from the camera.

From the two green lines on the photo, as well as the camera's field of view, you can determine the visual angle α0α0 of the slope as seen from the camera.Given a distance DD from the slope, you can determine the visual angle α(D)α(D) of the slope as seen from a camera at distance DD from the slope, since you know the position of that camera and both ends of the slope. Note that α(D)α(D) is strictly monotonous in DD. Your aim is now to find D0D0 such that α(D0)=α0α(D0)=α0. This can be numerically solved using half-interval search or Newton's method (Newton's method will converge if D0>0D0>0 and your initial value for DD is 00).