You need to cut past your pencil marks that are at the beginning and the end of the cut. Since this is a bevel cut, if you begin the cut at your pencil mark and end the cut at the other pencil mark, the blade will not go all the way through the mat board, leaving it attached at the corners. When that happens you will likely have corners that are torn. This being the case, if your corner cuts from the two directions do not overlap and the mat board in the window of the mat does not fall out freely, you will need to hand cut it, at the beveled angle, with a blade that is not in the mat cutter.
thanks John - very clear
Your term, "other side" is confusing to me. Do you mean "outside" your lines?
You need to cut past your pencil marks that are at the beginning and the end of the cut. Since this is a bevel cut, if you begin the cut at your pencil mark and end the cut at the other pencil mark, the blade will not go all the way through the mat board, leaving it attached at the corners. When that happens you will likely have corners that are torn. This being the case, if your corner cuts from the two directions do not overlap and the mat board in the window of the mat does not fall out freely, you will need to hand cut it, at the beveled angle, with a blade that is not in the mat cutter.