Electric field lines fall within a circle?

I was going through the method of images using 2 positive point charges placed along a line perpendicular to an infinite grounded conducting plate. I had found the surface charge density induced on the surface by these charges and also know that the electric field lines from either of the charges must fall within a circle (since the field lines diverge radially outward from the positive charge trying to end up in the negative image charge. However, I wish to derive the radius of the circle within which the electric field lines will always fall. How am I supposed to attack the problem?