Unless the Nav's changed (which - as my sainted grandmother used to say- "I kinda F-----g doubt it"), mechanics measure it with a micrometer, mark it with a piece of chalk, and cut it with an axe.
Changed a little, now we just eyeball it, walk off to get the reference material we should have had to begin with, come back and cut with dull chainsaw whereever we could get the lagging off easily and do a little praying that NRRO didn't see any of it. Oh yea afterwards, fill out all the required paperwork, including pre work authorization forms and tags(ok just kidding about the tags).