When I mounted a different lens on an old folding Polaroid many years ago, I put tracing paper where the film would lay (by disassembling an old 667 film packet). Then I unscrewed the reflector from my Mini Mag Light, leaving only the brightly lit grain-of-wheat halogen bulb visible and burnng. In a darkened room, I meticulously measured out selected distance from the film plane to the bulb, and then focused the lens until the image was sharp on the "ground glass". I then marked the focusing scale with a mark for that specific distance. Because the lens had been changed, I could't use the old focusing scale which was used pictograms anyway, but the new scale i put on top of the pictogram worked fine for my purposes. Of course, as you say, at close distances DOF is critical.