in the video, when he's first showing the dimensions, you can see the angle. just re-watched the video, and i get where you're having the problem. Have you cut out the back piece of the wing yet? the part that goes from the wing to being the tail? have a look at how wide that is. now put those marks on your foamboard. that way you'll have the flat section at each end of the wing (youll fold them over and they will line up) does this make sence?
so if the width of the back piece is 6 inches. find the centre line of your wing and mark 3 inches on both sides. then connect that mark to the mark you made on the side. this will give you the correct angle. (measure the back piece, this way you can get it exactly lineing up and looking sharp)