The mat could have slipped in the cutter sending the line off to the side somewhere. In that case just recut the offending mat. Now cut your foam core for the backing. Foam core is a compressed Styrofoam product and is close to acid free. Hint, if you want to send a real framer into an absolute tizzy just try to frame something on sticky board and see what new and interesting facial expressions you get from them.
Sticky board has acid materials and a really nasty glue on it.
This glue comes through the work and discolors it. Avoid it where you can. and use foam core. Trace one of the mats on the foam core and cut it with a large knife using the guide rail on the mat cutter for a straight edge. Mounting the Piece Lay the piece out over the foam core and put the undermat on top of it. Line these up to the approximate position they should end up in.