One of the many things I have learned in this hobby is when cutting styrene only cut once then snap along the line. You get a much straighter and cleaner line than trying to cut through the styrene and whats more it works. Another trick is to purchase a very cheap set of metal vernier calipers and file a point on the top two gauges (if not already done). This helps in making sure measurements are always as accurate as they can be as you can mark the styrene with the pointed ends.
Trust this is of some help.
Rod.