Focusing 'shift' for IR is important with LF's long lenses. I don't know how someone could find otherwise. Heck, using just a 25A on Pan film shifts focus. With your film, you are working in the low IR range - maybe around 750 or 800 - but still significant. I would have to find my old book, and will if you ask, but I seem to recall a figure of a 3% (to possibly 5%) extra bellows extension is needed.