If you haven't done it yet, and it's a straight line you need, snap it. Get a straight edge (metal, preferably) and run a blade down it a few million times lol. Don't put a lot of pressure on it, just enough to make a groove on the plexi. Then, flip it over, put the straight edge on the line, put some pressure on it (I usually use my knee), and pull up against the straightedge. The groove will act as a "starter" for you, and the plexi will snap in a clean straight line.