I had one plane with a Y normal. I wanted to change in into an infinitely long "runway."  I CSG'ed it with two planes with normals parallel to the X line.    Now I know I had to try several iterations of +x, -x, and / or +/- distance along normal before I got the strip to look that way I wanted it to look.

Is there a "wrong" way to get the right output?  If I had made an error (and I did several times), I would imagine I'd get something totally wrong looking, as opposed to an extra-slow-rendering version of exactly what I wanted.   Again, the render time was 2X for a CSG'ed plane (runway) than for a simple plane.
 

Ken wrote:

 Be careful about which way the surface normal is
pointing. It took me long time to get the hang of
it and I still find myself taking a look at the
different shapes inc files on occasion for ideas
and to figure out why something isn't working.
  Differences don't necessarily have to take a long
time to render.