From the patient input form, select "Customised" and the illustrated window will open.

Place an x-ray in front of the screen, or you can equally load-up a previously saved image (either a  latero-lateral or postero-anterior projection): if the "Angle" option has been selected, click three times with one of the mouse buttons to locate the three points that define the angle. The first point will be the vertex, the second point defines the first side and the third point the second side. (As is conventionally used, the values are considered as being taken anticlockwise).

To calculate the distance between two points, make the two sides of a triangle coincide, or better still, click the second and third points of the triangle on the same spot with the left then right mouse buttons.