Sorry for not replying before, I don't have much time in last week.
Also I'm only beginner in drawing and I know perspective more in theory than practice. Because of that, your suggestions are important.
This is top view of drawing scene, V is viewer, P is image plane, C, R and L are default points.
For each vanishing point are computed angle between forward (C) direction and vanishing point.
Thad angle is not linearly related to image coordinates, for example difference between 10 and 20 degrees VPs is much smaller than between 60 and 70 degrees.
This is also main reason for having default "special" points. This function will be much more visible after implementation of point grouping.
I don't want adding points automatically. User should be in control where to place new point.
Maybe making button to toggle button and allowing placing multiple VPs while it is pushed?
Added to todo list.
Yes, one of their roles is defining horizontal line.
In one next update could be hidden by default. There could be type of points that is not affected by these special pints.
As said @tilkau your idea of measurement axis could be used not only in connection with perspective mode.
This could be implemented as special case of grouping. Two pints could be chosen to define additional horizon line or measurement axis.