Attached Link: Example

My method is to use a cube, turned 45 degrees on the y axis. I then move the cube back and forth on the x & z axis and up and down on the y-axis until the front edge of the cube just intersects the point I want to focus on.

After that I move the cube only on the x and y axis to center it on the diagonal cross hairs. It's very important not move it forward and back on the z axis at this point - only left/right on the x axis and up/down on the y axis - and then use the cross hairs to get to correct focus distance by placing them on the edge of the cube (of course you should delete the cube or make it invisible before rendering...

Does that make sense - see the attached example - link above