[Leica] M's and focusing

[Yods1 <Yods1@aol.com>]

An open question regarding M's and lenses with good flatness of field--suppose
I'm shooting wide-open with a wide angle lens and my subject is, say, a person
who's pretty close, and I want the eyes to be sharp. I focus on the person's
eyes in the center of the finder, but that's not what I want in composition,
so I move that person close to the edge of the frame. Technically, it seems to
me that if the field-plane is truly flat, that will back-focus the eyes of the
person when I change the composition.
	Please bear with this ludicrous example of why I flunked math;
I cut out a triangle, representing the field of view of a wide-angle lens, and
kept one point pinned to its place. The center of the opposing side was where
I imaginarily focused on a subject, that side being the focal plane. When I
shift the triangle it covers the point of focus and seems to prove that it
would backfocus on sensitive, precise wide-aperture focusing.
I'm still trying to decide between R's and M's. 
Am I full of it? somebody let me know. Thanks...