Surely there has to be some computation of angles internally to identify the focusing plane in order to eliminate the back-focus.
Why does there have to be? The point is, with a fast lens shot wide open, don't focus/recompose. Use an outer focus point so you don't have to move the camera. With narrow apertures and/or distant subjects, the deeper DoF masks the backfocus, so focus/recompose is ok.
Sorry, I don't doubt that it needs to be done. The programmer in me is curious about how it is implemented thats all.