Stand at some location on the baseline. Hold box level with hole in front of your eye, and edge parallel to baseline. Look toward object and place the pin in your line of sight. Move a good distance along the baseline and repeat the sighting. It is not necessary that these be acute triangles.
Obtuse triangles work just as well.
You will be finding the distance, D, perpendicular to the baseline. You will need a table of tangents. You may call for such a table or make one for yourself.