Wear a mask. The nasty dust is the only real drawback.
I prefer to cut the Plascore before glassing. I let the glass run wild 1 or 2 inches past the edges of any panel I'm applying glass to.
If you deal with that extra glass too late it's a pain the ass. But if you come back a few hours later (how hot is it? Which hardener do you use?) when the glass is tack free but still soft and rubbery you can trim it with a sharp sheetrock knife. Lickedy split.