Hmm, wondering what the trouble is. I have cut 35mm backing paper for my 828 cameras from 120 rolls several times, and haven't had any issue except a little too much frame spacing (I use the numbers from the 6x4.5 / 16exp strip, you can get about 12 on an 828 spool). If you use the 12-number strip from the 6x6 exposures---wait, that's in the middle of the 120 roll, so you'd have to trim both sides. Is that the issue? A standard mat cutter gives ~30" of very straight cut, and if you slipped up around the beginning of the next 30", it'll be within the roll, so doesn't make too much difference.