Metered camera exposure is 8 / ISO. Opacity comes from the step tablet and incident light is based on the metered camera exposure. These are the two variables needed to calculate the transmitted light for a given step tablet density. Transmitted light = Incident Light / Opacity. The film speed equation is 0.80 / Hm. What the exposure needs to be for a certain film speed is 0.80 / ISO.
So, basically 1/10th the light arithmetically from the metered camera exposure... Or 1.0 logarithmically speaking, 3 1/3 stops down from the metered point. Was it just chosen to be easy to be remembered?