What you need to do is prevent the tube from pushing through the tire if the cut expands all the way through. That means the patch should be on the inside of the tire, not the outside. In an emergency on the road you can fold up a dollar bill so that it will fit inside the tire and cover the gash after you replace the tube or repair the hole in the tube. If you are going to do the 50 mile ride, you might use the dollar bill trick as a backup for the patch.