New Post: Removing white from image with white background where gradients are present

I'm working with images that were created as monochrome TIFFs (colour separations) where I need to convert them to PNGs with a transparent background, removing all the white.

The problem is, the images have gradients in them. So for example I need to be able to replace not just solid white with transparent (which I managed to do already) but also take into account percentages; so for example what was 40% black before would become 100% black but 40% opaque, if that makes sense.

Looking for recommendations as to how this could be achieved with Magick.NET?

Thanks in advance.