I have found, sadly, that if you don't have your digital Leica and your lenses calibrated every few years (depending on how much and how hard you use them) focus calibration wanders.
Okavango Delta - Leica MP 1/250, 35 Summilux ASPH @ f5.6 medium yellow filter, Tri-X in Xtol 1+3.
Focus shift occurs more in lenses with certain kinds of residual spherical aberrations. As the aperture changes, the point of focus changes (shifts).
Lens designers always have to make compromises. The distribution of the field of acceptable sharpness depends on the focal length of the lens. Shorter focal length lenses always have less depth of field in front of the subject than longer focal length lenses, but they have more depth of field behind the subject than longer focal length lenses. At close range and wide open the focus point of an 11874 as Leica designed it to be calibrated is ever-so-slightly in front of the point at which you focus. As you stop down the focus point shifts away from the film/sensor plane. When the shift is greater than the increase in depth of field this causes out-of-focus photos.
In practice with the 11874 if you avoid using the lens at distances <3m at f2.8 and 4 it will be a minimal problem. I understand that you want to use the lens on film, but if you have access to a digital M and a focus checker, you can take lots of photos fast at different apertures and distances with a focus checker like this: http://michaeltapesdesign.com/lensalign.html
and see the difference.
But enjoy. It's a great lens.