Hi Calico,
I might be wrong, but this is how I approach PFs (Powerfractals):
I get an overview of the object's size I want to texture (terrain, spheres, leaves).
This size will be my lead in scale (approx.).
The smallest scale is just how fine will the texture be in detail.
The feature scale for me is something that needs further explanation:
Make a feature scale the same size as the lead in. With Colour Roughness turned to 0 you will get some shallow, clean fractal.
Make a feature scale half the size of the lead in. With Colour Roughness turned to 0 you will get twice the noise. (Lead in is the maximum size of the noise, Feature scale is the distance between two noise patterns - or think basically of 100m sized hills [lead in] every 50m [feature scale]).
The colour roughness is giving this pattern the amount of variation that is sometimes needed. I reduce the roughness for blending shaders or for displacements. I push the roughness for surface patterns.
Colour Contrast just is the 'steepness' between lowest and highest colour and thus rather a fine tuning - or essential for getting smooth blend shaders.
So first I made a large scale fractal with contrasting colours and some medium roughness. Scale is from 3 to 100m, repeated every 19m. Just like large stone-like features.
The second one has a scale from 19 to 76, repeated after 17m (I just love multiples of 19,17,13 and so on). Less colour contrast but immense roughness. The detail comes from the blending shader at the edges. Like plaques sitting on the first PF.
The third one is even smaller. Interesting is the negative colour offset in the blending shader. Due to the low contrast there is no white but heaps of black holes. Instead I could have typed in high-colour 0.75 and low-colour -0.25 - and this what the clamp colour is useful for.
The choice of the colours themselves was just try and taste ,-)
I hope this helped, feel free to ask and/or correct me.
Volker