Roughly,

In determining coordinates, measure distance along lines that are parallel to the coordinate axes. The lines you use will form a rectangle or the sides of a box.

For example, in the figure, the black dotted lines used for point Q0 form a rectangle. Measuring the sides of the rectangle leads to the coordinates:

It is harder to figure out the coordinates of Q1. The picture is 2D, but the scene it shows is 3D, so the Z axis is distorted. The dotted lines in the 3D scene form a box. All the angles are 90 degrees.