I couldn't find much info on this so I sacrificed a roll and am posting what I found. I had heard you could get 9 shots on 120 in the 220 back but based on my test you can get 10 shots on 120 film in the 220 back.
Use "7" as your "LOAD". Roll the 120 film until the start arrow is visible on the takeup spool, facing the same direction as the lens. Advance to "10", this is your "1". "19" is your 10th image. (Using 6/9 for LOAD/1 also worked for me. 8/11 would probably be ideal since you would be ending at the same place as the back thinks you are but I haven't tried it).
Use "7" as your "LOAD". Roll the 120 film until the start arrow is visible on the takeup spool, facing the same direction as the lens. Advance to "10", this is your "1". "19" is your 10th image. (Using 6/9 for LOAD/1 also worked for me. 8/11 would probably be ideal since you would be ending at the same place as the back thinks you are but I haven't tried it).