This is my version of bb's ISF shader, with the nodes re-arranged so you can see what goes where. The values are what I used in that image. (Note: this shader is set up for use with inch units of measurement).

You have to plug in the XYZ location of the point light for the shader to work properly. With the XYZ values as Parmatic parameters, I can just copy them across on the Parameters palette after positioning the light.

You also have to specify the distance from the light at which the intensity is the level chosen on the Intensity dial. Again, I have this set up as a Parmatic parameter dial.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)