Hi BJ,
Yep new Stanley blade every time,i also put a wrap of masking tape around the area i'm going to cut to stop any chance of splintering.
It's a question of gently scoring as opposed to putting to much downward pressure which could crush the section.
After make sure you remove any sharp edges of carbon with some fine grade wet/dry paper or an emery board
Just make sure you trim off a bit at a time,you can always take a bit more off but you can't put it back on.
I never put an elastic larger than a 12 through my top kits so they are all fitted with bushes to fit a 14 elastic and trimmed to the same length.
I also trim the cupping kit to the same length to ensure accurate feeding.
Peter.
Make Friends,Go Maggotdrowning.