In order to tell where the vanishing point is one must lay a ruler over the lines, and the point will be where the lines meet. Some make a hole at the end of their rulers in order to fix the point by that hole while moving the ruler about. Others lay a ruler across the panel,, then on this ruler they place the tip of another ruler which, being attached to the former by a screw, will move about and reach as far as necessary without losing the point. The ruler and compass without discernment of the eye will cause errors. Perspective is very difficult.
at left middle margin of folio 062v
To paint a perspective in oil, you should not use a ruler, for you would smudge everything. Instead, in applying your colors you must keep and follow the lines of your first drawing.