How do I find the inclination of my Horizon line with respect to the paper/board while sketching a 2-point perspective?

Question

Premise: In a 2-point perspective, the two vanishing points dictate the angle of the lines parallel to the respective axes they represent, say X and Y, while the third axis, say Z, is perpendicular/orthogonal to the horizon line, i.e., the line connecting the two vanishing points.

Problem: When I am drawing a two point perspective and the vanishing points are well within the bounds of the paper, I am able to figure out the horizon line by connecting the vanishing points, and hence I know how much inclined/vertical the lines parallel to Z axis would be.

But when the vanishing points extend beyond the paper, I have no means to connect them as I don't effectively know where exactly they exist, and hence can't establish the horizon line or the angle it makes with the paper's edges.

In such a scenario, how am I supposed to draw the lines pertaining to the Z axis?

Answer 1

You need to know where the wanishing point is or you can not draw an accurate perspective. So regardless of wether it fits your paper or not you got to know its location.

Now nothing says your wanishing point has to be on your paper. There are a few ways you can deal with this. One is to put the wanishing point on another paper, prefeerably a bigger one, but it might ve just a gew post it notes too, many papers etc. The problem here is that you nolonger can move paper, unless you accurately mark the alignment. The second way is to reflect the lines from paper edges and get the virtual point inside the paper.

However if you are just eyeballig it (don't) why not start by defining horizon before you estimate the wanishing points.