Your “slope selector” controls your gradient, reduce falloff to zero, set it to linear for a good measure, then set your slope selection.
As for:
World machine works in linear color space, if you need to convert colorizer to 8 bit to set your color keys to that albedo value, first remap those values to linear 0-1, then input respective Black and White (0-1) “Physical albedo” value to those remapped numbers. Remember to export your bitmap into an 8 bit format, that should keep to the values you specified in your post. I’m bad at math, so you have to figure out that remapping on your own lol!