What is the optimum viewing distance for the KMQ prism viewer?

The mirror based over/under 3-D viewers can be manually adjusted to fit most viewing distances.
With the prism viewer KMQ SSG1b, its fixed optical spreading of left/right eyesights (19deg) means that you have to measure the vertical distance of the stereo pair and then multiply by 4.5 to get the optimum viewing distance (paper or screen to viewer).