Take a piece of 2" wide masking tape----I like the blue stuff----and put it on the fuse with the edge EXACTLY on the edge of the fuse where the cowling will mate up. Run it all the way up one side and over and down the other side.
You now will have a reference line----the far edge of the tape,2" back from the break line---- to measure from when the cowling is on, before you start to trim.
Also protects fuse from scuffing.
Mike