@grosgood I am still working through your post 
but I am working toward a more generalized approach / understanding to feathered-intensity masking. I picture the end goal might be to figure out how to do a M-like mask that can be centered about any intensity point (from the initial -luminance or whatever) of my choosing; and be able to decide the width and curve of the feathering. I wonder if that would have anything to do with your other post.