Finding the Area under a vertex paint stroke

Hi Everyone,

I have hit a bit of a material block. I am making a vertex paint material to control the normal things like levels of damage etc.

I need to find the -Z area of a vertex paint area.

For example, the image has two lines, the red is the vertex painted area. the lines in blue is the area i want to pull from the painted mask to add in leaking from the rust spots.

I’m having a bit of trouble figuring out if this is even possible. I have tried a couple of attempts but they never lead me anywhere close to what i’m trying to do.

I searched around and couldn’t find much on this topic so I was hoping someone would have thoughts.