I use the standard deviation slider to adjust scale. First I use small scale NR (around 0.8-1.2 px scale) with a very light mask for luminance to provide overall noise reduction to the image (chrominance around 1-1.6 px scale; no mask). Then, after resetting the black point, I apply noise reduction with a larger scale (4-6 px luminance; 5-7 chrominance) and use quite a dark mask (on both luminance and chrominance) to limit this step to the lowest signal regions of the image.
Clear skies,
Ron