i shorten mine with a bench grinder,,, i use the corner of the stone and roll the flex until it grinds through at the point... works for me,,,do it all the time.... if you use the corner of the stone it will grind through like this...
bench top sander with heavy grit paper like above roll it and its even better when doing it this way to solder it so the end dont fray---if you have much to remove cut the cable with a dremal or other cut-off wheel BUT leave it a little long, solder that end and hit it with your grinder sander etc to get it to your desired length
I wrap tape on the keeper side of the cut...then I tin the flex a little using flux and solder at the spot I taped. Then I cut through it using a cutting wheel on a dremel...and round off the edges. The solder wicks down into the wrapped end (keeper side) of the cable. If you get too much solder too far into the keeper side of the cable, it will brake when it flexes.
honestly,,, and this may sound wierd,,, ive never tinned the collet side of any cable ive cut and never had an issue....may be just me,, but thats the way ive done them for years..........
i had forgot to add also,,, when rolling the flex against the corner of the stone,,, make sure the wheel is spinning the same way the cable is wound on the outer layer,,, this will keep the wheel from unwrapping the end your cutting......