Unfortunately, it all goes over my head at the moment.
I was looking though, and you don't have to use a ray from the camera, if I remember right. One simple way might be to cast a ray straight down from above the terrain and move it around with the mouse. That way I would know the x,z from the ray position and I could just place some kind of marker for the cell size when it crossed to the next cell using the y position from the average of the vertices and the x,z from the ray start.