Phil, if I understand your question correctly, you are asking about placement of the sub during the sub crawl.
The ideal spot is "in" (on) your seat, if you can do that without destroying the seating. You want it be roughly where your ears would be so you can crawl around placing your ears where the sub would be. In essence, your ears and the sub are trading places temporarily so you can find the spot where the sub would produce the best bass in the seating area.
Next best place would be on the floor exactly where your seating would be, but you say that isn't possible. If the best you can do is place it just in front of the seating, then that is the best you can do. Place it in front of your seating, do the crawl, find the spot where you are hearing the smoothest, tightest bass, place your sub there, and don't worry about it.
When it comes to our sound systems, most of us in the real world have to make compromises. When setting up your system, do the best you can then sit back and enjoy the movies and the music. If you are listening to your system rather than to the movie or music, you are missing the point.