Just keep careful track of your frame count as you shoot so you don't go shooting on the tail leader with no film That's of course where your best shots will be lost.

BTW 120 in a 220 back needs to start at a differnt point, not the arrow on the leader, closer to the start by a couple inches. So take a paper backing n run it thorough n adjust your start point for the first frame to fall in the right place just past eh tape holding the film on the backing paper. I generally wind on one turn on the empty spool n that gives me a perfect frame one start, otherwise you'll start late on the film n loose one or 2 frames on the tail.

FYI.. the repair manual shows the only difference is in the frame counter parts not the rest of the operating parts in the 120 vs 220 backs... no springs n preasure plate differences, no focusing problems... all myth. Try, run the paper backing to test it n see for yourself.