There's more than one correct method, if you cut it in the right place that's correct!
Choices are - cut by measuring, either with one of the tools (there's been a couple over the years) or a ruler, the measurement will be there somewhere if you google it (I have an idea what it is but won't say in case it's wrong!) OR - cut it against the fitting coming from the hub subtracting 5mm for the other side of the bayonet fastener. I always used the latter method, it's more intuitive to me, the only thing to watch out for is you don't move the shifter while you're doing it, Thorn recommend taping it up, choose for yourself if you think that's necessary.
Main thing is to take care, but don't be too nervous of it, a few mm either way can be adjusted out, the one time I cut it short (Silly error, I subtracted the 5mm twice!) I just cut a bit off the outer and all was well.
