# How can I reduce the radius of this distance calculation?

I just created the scene shown in this "tutorial" (Is it possible to dynamically reduce the number particles based on distance from camera?), where you see how to dynamically reduce the number of the particles depending on the distance to the camera, but now i find, that the radius is too big and covers the whole plane. I would like to just have a part of the plane covered with grass, but it should follow the camera over the whole plane. Is this possible?

Thanks for answers (and sorry for my bad english, but I'm an austrian.)

maxDist = max( vert_distances )

maxDist = 5