Where does the side of the car stop an underside start?

[bobswei]

I'm getting ready to paint the underbelly of a 70 E and not clear where the edge is. I've searched thread after thread looking for photos of original cars but can't seem to find any. I've also seen a couple different opinions on what is the factory (Underside) color for a 1970. Black or grey / tan (undercoat color). At this moment, I'm going with "black" for underbelly, engine compartment and wheel wells.
Anyway, here is where I'm not sure of.... red line, blue line or somewhere in between?
Thanks