Leslie Stroebel recommends focusing, and then increasing the lens-to-film distance by 1/400th to correct the focus for infrared film.
Leitz once recommended a correction in the range of 1/200 to 1/300.


As there are quite a bit recommendations out there, the best way would be to fix several markings (strip of millimeter lined paper or so) for a IR-focussing mark at the barrel and to do test exposures at a critical target.