I made several measurements on stock spoilers before installing mine. First you should find the centerpoint of the trunk and place a piece of painters tape there with a line marking the center. I would also recommend just putting tape all the way accross the rear of the trunk lid. Then place your spoiler on the trunk with the back of the posts 1/8" from the rear of the trunk lid. Measure from the center point out to the posts and adjust spoiler until you get it even on both sides. Attach the drill guides to the car as the instruction sheet tells you, then set the spoiler to the side and grab your drill. Drill a small pilot hole first (you are drilling metal after all.) When your holes are drilled, remove all of your tape, bolt on the spoiler, and step back to enjoy the view!