I guess it is because have different numbers for the lengths of 120 film (815mm) and 127 film (650mm) - a difference of 160mm, not 120mm.
But thanks for clarifying that the first cut is the lengthwise slit.
Don_ih describes what I'm doing correctly. Just for clarity here is a photo of the rig I'm using. It is made from an old 120 folding Brownie (the bellows were rotten and the lens is in another project. I didn't destroy a thing of beauty for this!). There are various completely 3D print machines or completely handcrafted machines on the interwebs.
I slice the roll and backing paper completely to 46mm leaving a ~15mm strip that I discard. I could put it in a Minolta 16 cartridge but shooting 16mm really doesn't spark joy.
The actual film of 120 seems to vary in length a bit between brands. I roll till I reach the correct point on the 127 backing paper (marked with a tape or punch hole) then cut off the remainder and taped down the end. The excess is indeed up to 200mm long - which also might vary with my rolling of the 127.
I have also made a stencil so I can mark the backing paper correctly with a sharpie for use in cameras with peehole windows but these aren't needed on a Rolleiflex because it is all automatic.
I have tried rolling the offcuts into short 127 rolls. The trouble doing this for any camera is lining up the short piece of film with the numbered backing paper. This isn't an issue with the Rollei because it detects the beginning of the film (then leaves an annoying gap). I can get between 3 and 4 exposures on a short using this method but, because you have to be particularly careful changing in subdued light with these hand rolled films, changing film every 3 exposures is a pain!
Everyone seems to do the slicing and rolling of 127 slightly differently. I should document my whole process I guess but I'd like to refine it a bit more first.
Sorry too much detail but I guess you are nerds for this stuff!!
I did some messing with 4x5 and CS glue last night and think I have a system to mount remains in 4x5 holders. I'll post it here when I get it to work.
(The "59.5" is not film width that is the offset of the blade. I had to print several holders till I got it just right.)