Using the guide block method should help greatly. For freehand I’ll mark my line across the board and down the sides. On the offhand side I’ll make a shallow cut about a quarter inch making sure to keep to both lines. I like to leave the lines showing. That cut will help keep your blade perpendicular. As I move along the cut I’ll now and then lower the handle to cut along the line to kind of score it, giving the blade a place to fall into so to speak. Letting the blade do the work without force allows the tailing end to follow along in perpendicular fashion. I’ve had pretty darn good results using this method.