Be careful here! First of all, is the lens focussing correctly at infinity. If so, does it back focus or front focus close up? If it is back focussing, than you do need to shorten the optical block slightly. However, if it is front focussing because someone has done a relube and put the wrong shims in, then it's the other way round. 😉 Any adjustment here wil affect the focus at infinity so that needs to be adjusted again. However, before diving into the shims, you do need to make sure any relube has been dome properly and the lens put back together correctly. Make sure you sort this first. As has recorded elsewhere, if you take a J9 focos unit apart and put it back together again, it can have an effect on the throw of the cam due to the tolerances in the manufacture. Redoing this several times can bring a lens "back in" that was correct before and save a great deal of effort with the shimming. Certainly, with the J9, you should not to file anything within the lens unit. Some J3's may need it. With the J9, if it is a long way out, you may need to thin the aperture ring slightly if you have had to move the lens unit by any sgnificant amount. This tends to be only with the black ones where they tried to "close the gap" to avoid too much chrome showing.
Kim