Ok I have been very unclear of how SSS work so I finally did some testing.
Here is my results and conclusions.
Please feel free to put me straight where I am wrong.
I made an object that is 2m long with a square either end 100mm the side linearly taper down to 1mm square at the centre. The light is directly opposite to the camera with the object in the middle.
As you can see by the diagram each layer penetrates so far though the object.
Colour input deliniates the colour you start with.
Scale specifies how deep the SSS penetrates in blender units.
Radius specifies how much of each RGB in the starting colour travels as a proportion of the scale.
The deepest layer with a scale of .06 has no blue in the original colour so giving it a full 1 of radius means nothing 1 x 0 is still 0.
The reverse obviously works as well.
This layer also penetrates 60 mm through the object (can’t really see the last bit cause it’s too dark but I presume it’s there) ie. 60 % along the object being that it has a 1:100 grade
Any more clarification would be gratefully received.