ORIGINAL: safeTwire
IMO as help is a "quality" metal straight edge long enough as reqiured. ( it will cost some sheckels ). Also, the surface underneath the wood your cutting can come into play. A trip to your lumber outlet for a sacrificial clean, flat and firm board will help.
If you go to a machining supply for a straight edge it will cost you about $100 for about 3 feet.
If you go to your local lumber supply or Walmart, you can get a Stanley metal yardstick for under $4 which is straight enough for anything that we use it for. Just make sure to get the bare metal yardstick and not one of the painted ones.